Braking system including motor-driven disc brake equipped with self-servo mechanism

ABSTRACT

Electrically operated braking system with a motor-driven disc brake including an electric motor for braking an automotive vehicle wheel, and a motor control device controlling the electric motor, the disc brake further including a disc rotor having a friction surface, a friction pad movable for contact with the friction surface to restrict rotation of the disc rotor, a pad support mechanism for supporting the friction pad movably in a direction intersecting the friction surface, a pad pressing mechanism including the electric motor and a pressing member, the electric motor producing a drive force for moving the pressing member to force the friction pad against the disc rotor, and a self-servo mechanism for providing a self-servo effect of boosting a friction force generated between the friction surface and the friction pad, on the basis of the friction force.

TECHNICAL FIELD

The present invention relates in general to an electrically operatedbraking system having a motor-driven disc brake activated by an electricmotor for braking a wheel of an automotive vehicle. More particularly,this invention is concerned with improvements in techniques for enablingthe motor-driven disc brake to produce an increased wheel braking force,without increasing a drive force or torque to be generated by theelectric motor.

BACKGROUND ART

In such an electrically operated braking system, there has been a needor desire to increase the wheel braking force for a given drive force ortorque generated by an electric motor used for the disc brake.JP-U-5-22234 proposes a conventional braking system, which is arrangedin an attempt to meet the above-indicated need. In this conventionalbraking system, a boosting mechanism is provided between an electricmotor and friction pads of a disc brake, so that the drive forcegenerated by the motor is boosted by the boosting mechanism before it istransmitted to the friction pads. For producing a relatively large wheelbraking force, however, the motor and the boosting mechanism in thisconventional braking system are subject to a comparatively large load,and therefore tend to have relatively large sizes, leading to anaccordingly increased size of the motor-driven disc brake.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide anelectrically operated braking system wherein a motor-driven disc brakefor braking a vehicle wheel is capable of producing a sufficiently largewheel braking force without increasing a nominal capacity of an electricmotor, by effectively utilizing a friction force generated by a frictionpad and a disc rotor upon braking of the wheel, while avoiding anincrease in size of the disc brake.

The above object may be achieved by any one of the following features ofthe present invention, which are numbered like the appended claims, soas to show possible combinations of the features:

(1) An electrically operated braking system comprising a motor-drivendisc brake including an electric motor as a drive source for braking awheel of an automotive vehicle, and a motor control device forcontrolling the electric motor, the motor-driven disc brake furtherincluding: (a) a disc rotor having a friction surface and rotating withthe wheel; (b) a friction pad movable for contact with the frictionsurface to restrict rotation of the disc rotor; (c) a pad supportmechanism for supporting the friction pad such that the friction pad ismovable in a direction intersecting the friction surface; (d) a padpressing mechanism comprising the electric motor and a pressing member,the electric motor producing a drive force for moving the pressingmember to force the friction pad against the friction surface of thedisc rotor; and (e) a self-servo mechanism for providing a self-servoeffect of boosting a friction force generated between the frictionsurface and the friction pad, on the basis of the friction force.

In the braking system of the present invention, the self-servo mechanismis adapted to boost the friction force and apply the boosted frictionforce to the friction pad. The friction force thus boosted by theself-servo mechanism with a given drive force of the electric motor islarger than the friction force which is generated with the same driveforce of the electric motor when the self-servo mechanism is notprovided. In this braking system, therefore, the load acting on theelectric motor is reduced, and the size and capacity of the motor areaccordingly reduced, as compared with those of the motor in theconventional braking system wherein only the drive force of the motor isused to press the friction pad against the disc rotor, without utilizinga self-servo effect of the friction pad based on the friction forcegenerated by the motor. Accordingly, the motor-drive disc brake can bemade small-sized and comparatively easily installed on the vehicle body.

The present electrically operated braking system can be used as anordinary brake for braking a running vehicle. In this case, the brakingsystem may be adapted to effect automatic control of braking forces tobe applied to vehicle wheels, such as anti-lock and traction controls.

The electric motor used in the present braking system may be awound-rotor type motor, or an ultrasonic motor. The holding torqueproduced by the ultrasonic motor in its non-energized off state islarger than that produced by the wound-rotor type motor. In thisrespect, the braking system using the ultrasonic motor can be suitablyused as a parking brake. In this case, the braking force required forholding the parked vehicle stationary can be obtained with acomparatively small amount of electric power consumption.

The motor-driven disc brake may use a pair of friction pads which aredisposed on the opposite sides of the disc rotor and which are forcedagainst the opposite friction surfaces of the disc rotor. In this case,the self-servo mechanism may be adapted to use the friction forcebetween the disc rotor and one of the two friction pads, for providingthe self-servo effect with respect to either the above-indicated onefriction pad or the other friction pad.

(2) An electrically operated braking system according to the feature(1), wherein said pad pressing mechanism includes a first pressingdevice for generating a first pressing force for pressing the frictionpad against the disc rotor based on the drive force of the electricmotor, and the self-servo mechanism includes a second pressing devicefor generating a second pressing force for pressing the friction padagainst the disc rotor based on the friction force which is generatedbetween the friction pad and the disc rotor based on the first pressingforce.

In the above braking system, the first and second pressing forces may betransmitted to the friction pad through respective different paths or asingle path.

(3) An electrically operated braking system according to the feature (1)or (2), wherein the motor-driven disc brake includes a pair of frictionpads disposed on the opposite sides of the disc rotor, respectively, andthe pad pressing mechanism and the self-servo mechanism include a levercorresponding to each of at least one of the friction pads, the leverincluding (a) a first connecting portion at which the lever is connectedto a stationary member such that the lever is pivotable about a firstaxis perpendicular to an axis of rotation of the disc rotor, (b) a firstbearing portion which receives the friction force generated between thecorresponding one of the friction pads and the disc rotor when thevehicle is running in a predetermined first direction which is one of aforward and a reverse running direction of the vehicle, (c) an engagingportion which is engageable with a back surface of the above-indicatedcorresponding friction pad, and wherein the first connecting portion,the first bearing portion and the engaging portion are positionedrelative to each other such that the friction force received by thefirst bearing portion from the above-indicated corresponding frictionpad causes a moment to act on the lever in a direction that causes theengaging portion to approach the disc rotor.

In the above braking system, the lever constitutes a major part of theself-servo mechanism.

In one form of this braking system, the first bearing portion (effortpoint), the relative position (lever ratio) of the first connectingportion (fulcrum) and the engaging portion (load point) is determined sothat the friction force generated between the friction pad and the discrotor is boosted and transmitted to the friction pad. According to thisarrangement, the self-servo mechanism achieves an effective self-servofunction to boost the friction force. In another form, the level has aninput portion at which the drive force of the electric motor isreceived. In this case, the single lever permits the first and secondpressing forces to be transmitted to the friction pad, and themotor-driven disc brake can be small-sized, as compared with the discbrake wherein two levers are used for transmitting the first and secondpressing forces, respectively.

(4) An electrically operated braking system according to the feature(3), wherein the pad pressing mechanism and the self-servo mechanisminclude the above-indicated lever for each of the pair of friction pads,and a pair of links connecting the two levers disposed on the oppositesides of the disc rotor, the pair of links being connected to each othersuch that the links are pivotable about a second axis parallel to theabove-indicated first axis, each of the links including (d) a secondconnecting portion connected to the corresponding one of the levers suchthat the link is pivotable about a third axis parallel to the secondaxis, and (e) a second bearing portion which receives the friction forcegenerated between the above-indicated corresponding friction pad and thedisc rotor when the vehicle is running in a second direction opposite tothe above-indicated first direction, and wherein the second connectingportion, the second bearing portion, the first connecting portion andthe engaging portion are positioned relative to each other the frictionforce received by the second bearing portion from the correspondingfriction pad causes a moment to act on the corresponding lever in adirection that causes the engaging portion to approach the disc rotor.

In the above braking system, the self-servo mechanism is able to achievethe self-servo function during running of the vehicle in not only theforward direction but also the reverse direction. Thus, the brakingsystem provides a sufficient wheel braking force irrespective of therunning direction of the vehicle.

(5) An electrically operated braking system according to any one of thefeatures (1)-(4), wherein the motor-driven disc brake further includesself-servo effect inhibiting mechanism for inhibiting the self-servomechanism from providing the self-servo effect while a braking forcebetween the wheel and a road surface is smaller than a predeterminedfirst value.

The self-servo effect is advantageous in that the friction forceeventually generated between the friction pad and the disc rotor islarger than the drive force of the electric motor. At the same time, theself-servo effect has a disadvantage. That is, the boosting ratio (gain)of the drive force of the electric motor, which is a ratio of theeventual friction force to the drive force, tends to be excessivelylarge, and the actual wheel braking force tends to be excessivelyresponsive to the drive force of the electric motor. Further, theeventual friction force tends to increase non-linearly with an increasein the drive force. It is also noted that the self-servo effect islikely to be influenced by a change in the friction coefficient of thefriction pad. This disadvantage of the self-servo effect leads toinstability in the braking effect provided by the disc brake. On theother hand, the need for the self-servo effect varies depending upon thebraking condition. Namely, the need for the self-servo effect isrelatively low during application of a normal or ordinary brake to thevehicle, and is relatively high during application of an abrupt brake.In addition, the normal brake application requires high stability in thebraking effect, while on the other hand the abrupt brake applicationrequires a maximum braking effect to provide a sufficiently large wheelbraking force.

In the light of the above, the braking system according to the abovefeature (5) is adapted to selectively provide the self-servo effect onlywhen this effect is needed.

That is, the disc brake includes the self-servo effect inhibitingmechanism which is adapted to inhibit the self-servo mechanism fromproviding the self-servo effect as long as the wheel braking forcebetween the wheel and the road surface is smaller than the predeterminedfirst threshold value.

In the above arrangement, the self-servo effect is not provided when itis not required, for example, upon normal brake application in which therequired wheel braking force is relatively small. Accordingly, thepresent arrangement assures high stability in the braking effect duringthe normal brake application. Upon abrupt brake application in which therequired wheel braking force is relatively large, on the other hand, theself-servo effect is provided to increase the braking effect with awheel braking force larger than the drive force of the electric motor.

The first threshold valve may be an upper limit of a normal range of thewheel braking force. The normal range is defined as a range within whichthe wheel braking force is expected to fall during normal brakingapplication (with a normal brake operating force). Alternatively, thefirst threshold value may be the wheel braking force which is expectedto be produced when the vehicle deceleration is in a range of 0.5-0.6 G.

The self-servo effect may be provided by utilizing a dragging movementof the friction pad with the disc rotor due to the frictional contacttherebetween. In this case, self-servo effect inhibiting mechanism maybe adapted to either mechanically or electrically inhibit the draggingmovement of the friction pad with the disc rotor.

(6) An electrically operated braking system according to the feature(5), wherein the self-servo mechanism provides the self-servo effect byutilizing a movement of the friction pad with the disc rotor due to thefriction force therebetween, and the self-servo effect inhibitingmechanism includes an elastic member whose elastic force inhibits themovement of the friction pad with the disc rotor.

(7) An electrically operated braking system according to the feature(6), wherein the self-servo mechanism provides the self-servo effectsuch that the self-servo effect changes with an amount of the movementof the friction pad with the disc rotor, and the elastic force of saidelastic member increases non-linearly with an increase in an amount ofelastic deformation of the elastic member.

The elastic member may be adapted to produce a predetermined constantelastic force irrespective of a change in the amount of elasticdeformation. In this case, the predetermined constant elastic forcedetermines the moment at which the movement of the friction pad with thedisc rotor is permitted, that is, the moment of initiation of theself-servo effect. Alternatively, the elastic member may be adapted suchthat the elastic force increases with an increase in the amount ofelastic deformation. In this case, the elastic member makes it possibleto control not only the moment of initiation of the self-servo effect,but also the rate of increase of the self-servo effect (i.e., boostingratio of the drive force of the electric motor). By suitably determiningthe relationship between the elastic force and the amount of elasticdeformation of the elastic member, that is, by optimizing the elasticcoefficient of the elastic member, the elastic member can achieve thefollowing two functions: (a) to permit the initiation of the self-servoeffect when the friction force is relatively small, and (b) to preventan excessive rise of the rate of increase of the self-servo effect afterthe initiation of the self-servo effect. The first function (a) may beachieved by reducing the force to be transmitted from the elastic memberto the friction pad, to permit the movement of the friction pad with thedisc rotor when the friction force is relatively small. The secondfunction (b) may be achieved by increasing the force to be transmittedfrom the elastic member to the friction pad, to restrict an increase inthe speed of movement of the friction pad with the disc rotor.

In the braking system according to the feature (7) developed in thelight of the above, the elastic member is arranged such that the elasticforce increases with an increase in the amount of elastic deformation.In this braking system, not only the moment of initiation of theself-servo effect but also the rate of increase of the self-servo effectcan be controlled as desired. The rate of decrease of the self-servoeffect may also be controlled.

The self-servo mechanism may be provided by the friction pad whichfunctions as a wedge to provide the self-servo effect. In this case, theelastic force of the elastic member to be applied to the friction padmay be constant irrespective of a change in the amount of elasticdeformation of the elastic member. In this instance, the rate ofdecrease of the wheel braking force may be excessively high uponreleasing of the brake (brake operating member), for the reason whichwill be apparent from the detailed description of the preferredembodiments of this invention. The braking device according to thefeature (7) is effective to not only facilitate the initiation of theself-servo effect, but also prevent the excessively high rate ofdecrease of the wheel braking force by increasing the elastic force ofthe elastic member.

(8) An electrically operated braking system according to the feature(7), wherein the elastic force of the elastic member increases linearlywith an increase in the amount of elastic deformation.

(9) An electrically operated braking system according to the feature(5), wherein the self-servo mechanism provides the self-servo effect byutilizing a movement of the friction pad with the disc rotor due to thefriction force therebetween, such that the self-servo effect changeswith an amount of the movement of the friction pad, and the self-servoeffect inhibiting mechanism includes an elastic member whose elasticforce inhibits the movement of the friction pad with the disc rotor, theelastic force increasing non-linearly with an increase in an amount ofelastic deformation of the elastic member.

In the above braking system, the elastic member can achieve variousfunctions including those achieved by the elastic member according tothe feature (8).

(10) An electrically operated braking system according to the feature(9), wherein a rate of increase of the elastic force of the elasticmember with the amount of elastic deformation is higher when the amountof elastic deformation is relatively large than when the amount ofelastic deformation is relatively small.

The above-indicated rate of increase of the elastic force with theamount of elastic deformation represents the elastic coefficient of theelastic member. In the above braking system, the elastic coefficient ishigher when the amount of elastic deformation is relatively large thanwhen the amount of elastic deformation is relatively small. Thisarrangement not only facilitates the initiation of the self-servo effectbut also prevents an excessive rise of the rate of increase of theself-servo effect during activation of the self-servo mechanism. Wherethe self-servo mechanism utilizes the wedge effect of the friction pad,the present arrangement is also effective to prevent an early excessiveincrease in the rate of decrease of the self-servo effect.

(11) An electrically operated braking system according to any one of thefeatures (1) through (10), wherein the motor-driven disc brake furtherincludes a mechanism for mechanically controlling a rate of change ofthe self-servo effect of the self-servo mechanism with a change in thedrive force of the electric motor.

While the rate of change of the self-servo effect tends to beexcessively high, the mechanism used in the above braking system isadapted to mechanically control the rate of change of the self-servoeffect. This mechanism is effective to prevent an excessively high rateof increase of the self-servo effect.

In one form of the mechanism for mechanically controlling the rate ofchange of the self-servo effect, the elastic coefficient of the elasticmember is optimized so as to suitably control the rate of increase ofthe self-servo effect. In another form of the mechanism, the contactsurface of the friction pad which contacts the pressing member of thepad pressing mechanism is inclined with respect to the friction surfaceof the disc rotor, and the angle of inclination of this contact surfacewith respect to the friction surface is optimized so as to suitablycontrol the rate of increase of the self-servo effect. In a further formof the mechanism, the elastic coefficient of the elastic member isoptimized so as to suitably control the rate of decrease of theself-servo effect.

(12) An electrically operated braking system according to any one of thefeatures (1) through (11), wherein the pad support mechanism includes astationary member having a pair of portions for supporting the frictionpad at opposite end portions thereof which are opposite to each other ina rotating direction of the disc rotor, and the elastic member havingopposite end portions one of which is associated with one of theopposite end portions of the friction pad toward which the friction padis moved with the disc rotor during forward running of the automotivevehicle, the other of the opposite end portions of the elastic memberbeing associated with one of the pair of portions of the stationarymember which is nearer to the above-indicated one end portion of thefriction pad than to the other end portion.

In the above braking system, the stationary member may be a mountingbracket fixed to the body of the automotive vehicle, and the pair ofportions may be a pair of bearing portions which receive forces from therespective opposite end portions of the friction pad based on thefriction force between the friction pad and the disc rotor when thevehicle is running in the forward and reverse (rearward) directions,respectively. The end portions of the friction pads are opposite to eachother in the rotating direction of the disc rotor.

(13) An electrically operated braking system according to any one of thefeatures (1) through (11), wherein the pad support mechanism includes astationary member having a pair of portions for supporting the frictionpad at opposite end portions thereof which are opposite to each other ina rotating direction of the disc rotor, and the elastic member havingopposite end portions one of which is associated with one of theopposite end portions of the friction pad toward which the friction padis moved with the disc rotor during forward running of the automotivevehicle, the other of the opposite end portions of the elastic memberbeing associated with one of the pair of portions of the stationarymember which is remote from the above-indicated one end portion of thefriction pad.

In the above braking system, one of the opposite end portions of theelastic member is associated with the friction pad while the other endportion of the elastic member is associated with the stationary member,as in the braking system according to the feature (12). However, theelastic member in this braking system can be more easily installed inthe motor-driven disc brake, because the above-indicated end portion ofthe elastic member is differently associated with the stationary member.

Described in detail, the braking system according to the feature (12) isarranged such that the above-indicated one end portion of the elasticmember is associated with one of the opposite end portions of thefriction pad toward which the friction pad is moved with the disc rotorduring the forward running of the vehicle, while the other end portionof the elastic member is associated with one of the pair of portions ofthe stationary member which is nearer to the above-indicated one endportion of the friction pad. In this arrangement, the distance betweenthe opposite end portions of the elastic member is comparatively small.Where the elastic member is formed from a rod or sheet, the elasticmember is required to U-shaped with a pair of arms opposed to each otherwith a relatively small spacing, so as to permit a sufficient amount ofelastic compression of the elastic member. The U-shaped elastic memberrequires a relatively large space for installation in the disc brake,whereby the size of the disc brake is likely to be increased.

In the braking system according to the feature (13), one of the endportions of the elastic member is associated with the end portion of thefriction pad toward which the friction pad is moved with the disc rotor,as in the preceding braking system. However, the other end portion ofthe elastic member is associated with one of the pair of portions of thestationary member which is remote from the above-indicated one endportion of the friction pad. In this arrangement, the distance betweenthe opposite end portions of the elastic member is comparatively long,and therefore the rod or sheet of the elastic member is not required tobe U-shaped to provide the sufficient amount of elastic deformation.Therefore, the elastic member in the form of the rod or sheet requires arelatively small space for installation in the disc brake, which spaceis available without increasing the size of the disc brake. In otherwords, the space normally available in a disc brake can be utilized forinstalling the elastic member.

In the braking system according to the above feature (13), the elasticmember can be comparatively easily installed in the disc brake. Theelastic member may take the form of a rod or sheet whose major partextends along a straight line or a curve or arc. Alternatively, themajor part of the elastic member in the form of a rod or sheet may becorrugated and extend generally along a straight line or an arc.

(14) An electrically operated braking system according to the feature(13), wherein the disc brake includes a pair of friction pads disposedon opposite sides of the disc rotor, respectively, and the pad pressingmechanism includes (a) a caliper which extends over a periphery of thedisc rotor and engages said pair of friction pads and which is movablein the direction intersecting the friction surface of the disc rotor,the caliper comprising a reaction portion engageable with one of thefriction pads, and a presser portion for pressing the other of thefriction pads against the friction surface, and (b) a presser rod whichis supported by the presser portion such that the presser rod is movableby the drive force of the electric motor in the direction intersectingthe friction surface, the caliper functioning as the pressing member forthe above-indicated one of the friction pads, while the presser rodfunctioning as the pressing member for the other of the friction pads,and wherein the stationary member includes a bridging portion connectingthe above-indicted pair of portions, the elastic member extendingsubstantially in parallel with the bridging portion.

Generally, a stationary member for supporting a pair of friction pads ina disc brake has a bridging portion. In this type of disc brake, theelastic member according to the feature (13) can be disposedsubstantially in parallel with the bridging portion of the stationarymember.

However, the bridging portion of the stationary member is not essential,and may be replaced by the elastic member. In this case, the spacerequired for the elastic member is further saved.

(15) An electrically operated braking system according to any one of thefeatures (1) through (14), wherein the motor-driven disc brake furtherincludes an excessive self-servo effect inhibiting mechanism forinhibiting an increase of the self-servo effect of the self-servomechanism after a braking force between the wheel and a road surfaceexceeds a predetermined second value.

In the electrically operated braking system having the self-servofunction according to the principle of the present invention, thefriction force generated between the friction pad and the disc rotor isboosted by this friction force per se. To prevent an excessive increaseof the self-servo effect, however, it is desirable to positively ormechanically limit the degree of the self-servo effect at an appropriatepoint of time after the initiation of the self-servo effect. In the discbrake of the type adapted to provide the self-servo effect by utilizingthe wedge effect of the friction pad, for instance, the friction pad maybe squeezed by and between the disc rotor and the pressing member withan excessively large force (friction force) due to an excessive increaseof the self-servo effect, resulting in a sticking tendency of thepressing member to the friction pad, which leads to a possibility thatthe friction pad cannot be rapidly or smoothly returned to thenon-operated position upon releasing of the brake application.

In the light of the above fact, the braking system according to thefeature (15) was developed in an effort to prevent an excessive increaseof the self-servo effect.

In this braking system, the excessive self-servo effect inhibitingmechanism is provided to inhibit an increase of the self-servo effectafter the braking force between the wheel and the road surface exceedsthe predetermined second threshold value.

According to the above feature (15) of this invention, the self-servomechanism is prevented from increasing the self-servo effect beyond agiven upper limit. As a result, the tendency of sticking of the pressingmember to the friction pad can be prevented even where the self-servoeffect is provided by the wedge effect of the friction pad. Thus, thepresent braking system does not suffer from the problem that thefriction pad cannot be rapidly returned to the non-operated positionupon releasing of the brake operating member.

The phrase “inhibiting an increase of the self-servo effect” isinterpreted to mean complete inhibition of the increase of theself-servo effect, or partial inhibition and partial allowance of theincrease of the self-servo effect.

The excessive self-servo effect inhibiting means may be adapted toinhibit a further movement of the friction pad with the disc rotor, tothereby inhibit a further increase of the self-servo effect. In thisform of the invention, the further movement of the friction ad with thedisc rotor may be either mechanically inhibited or electricallyinhibited.

(16) An electrically operated braking system according to the feature(15), wherein said self-servo mechanism provides the self-servo effectby utilizing a movement of the friction pad with the disc rotor due tothe friction force therebetween, such that the self-servo effect changeswith an amount of the movement of the friction pad, and wherein theexcessive self-servo effect inhibiting mechanism includes a stationarystop for abutting contact with the friction pad to thereby inhibit themovement of the friction pad with the disc rotor.

In the above braking system, the stationary stop may be provided on amounting bracket fixed to the vehicle body.

(17) An electrically operated braking system according to any one of thefeatures (1) through (16), wherein the motor-driven disc brake furtherincludes temperature rise restricting means for restricting a rise of atemperature of the electric motor.

In the electrically operated braking system wherein the electric motoris used as the drive source, the temperature of the electric motor mayrise due to heat generated by the friction between the friction pad andthe disc rotor, or due to heat generated by a coil of the motor per se.The temperature rise of the motor (in particular, of its coil) may causeoperating instability of the motor.

In the light of the above fact, the braking system according to thefeature (17) was developed to improve the operating stability orreliability of the system.

That is, the temperature rise restricting means is provided forrestricting a rise of the temperature of the electric motor.

In this braking system, the operating stability of the motor is notdeteriorated due to the temperature rise of the motor, so that theoperating reliability of the system is improved even though the electricmotor is used as the drive source.

(18) An electrically operated braking system according to the feature(17), wherein the temperature rise restricting means is provided in apower transmission path between the electric motor and the friction pad,to restrict a transfer of friction heat generated between the frictionpad and the disc rotor, to the electric motor through the powertransmission path.

In the above braking system, the temperature rise restricting means isrelatively simple in construction for restricting the heat transfer tothe motor to restrict the temperature rise of the motor.

(19) An electrically operated braking system according to the feature(18), wherein the self-servo mechanism includes means for positivelyallowing the friction pad to be moved with the disc rotor due to thefriction force between the friction surface and the friction pad, forthereby enabling the friction pad to function as a wedge between thedisc rotor and the pressing member, to provide the self-servo effect ofboosting the friction force.

In the above braking system, a pair of friction pads may be disposed onthe opposite sides of the disc rotor. In this case, the electric motoris provided on one of the opposite sides of the disc rotor to move thepressing member for pressing the corresponding one of the friction padsagainst the disc rotor. In this arrangement, the term “pressing member”is interpreted to mean a member for transmitting the drive force of themotor to the above-indicated one friction pad. However, the term may beinterpreted to mean not only the member for transmitting the drive forceto the above-indicated one friction pad, but also a member fortransmitting the drive force to the other friction pad.

(20) An electrically operated braking system according to the feature(19), wherein the motor-driven disc brake further includes frictionresistance reducing means for reducing a friction between the frictionpad and the pressing member.

In the above braking system, the friction resistance reducing means isprovided for reducing the friction between the friction pad and thepressing member.

The friction resistance reducing means is effective to avoid a problemthat the movement of the friction pad with the disc rotor is disturbedby the friction between the friction pad and the pressing member.Accordingly, the friction resistance reducing means permits theservo-effect mechanism to operate in an efficient fashion.

In one form of the braking system according to the above feature (20),the friction resistance reducing means includes a thrust bearingmechanism provided between and in contact with the friction pad and thepressing member. The thrust bearing mechanism may incorporate at leastone rolling element such as balls or rollers which are held in rollingcontact with the friction pad and the pressing member. In another formof the same braking system, the friction resistance reducing meansincludes a material which has a relatively low friction coefficient andwhich is provided on at least one of the two contacting surfaces of thefriction pad and the pressing member. Alternatively, the frictionresistance reducing means may be provided by forming a plurality ofsubstantially parallel grooves or protrusions on at least one of the twocontacting surfaces.

(21) An electrically operated braking system according to the feature(19) or (20), wherein the self-servo mechanism includes the friction padwhich has a slant surface for contact with the pressing member, and theslant surface is inclined with respect to the friction surface of thedisc rotor such that a distance between the slant surface and thefriction surface of the disc rotor increases in a direction in which thefriction pad is moved with the disc rotor due to the friction forcetherebetween.

In the above braking system, the friction pad has a slant surface whichis inclined with respect to the friction surface of the disc rotor. Inother words, the surface of the friction pad for contact with thepressing member is inclined with respect to the friction surface of thedisc rotor. This contact surface of the friction pad can be inclinedwith respect to the friction surface by providing the friction pad witha slant surface which may or may not contact the pressing member, thatis, a slant surface which contacts either the pressing member or thefriction surface of the disc rotor. This aspect will be described in thecase of the friction pad which consists of a friction member having afront surface for contact with the disc rotor and a backing platesecured to a back surface of the friction member.

In the above case, the backing plate of the friction pad has a backsurface for contact with the pressing member, and this back surface isthe above-indicated contact surface of the friction pad with respect tothe pressing member. In this case, the back surface of the backing plateis inclined with respect to the friction surface of the disc rotor wherethe friction member has a constant thickness in the direction ofmovement of the friction pad with the disc rotor, while the backingplate has a thickness which continuously changes in the above-indicateddirection. According to this arrangement, the back surface or contactsurface of the backing plate is inclined with respect to the front andback surfaces of the friction member, and functions as the slant surfaceinclined with respect to the friction surface of the disc rotor. In thiscase, the back surface of the backing plate is recognized as an inclinedsurface of the friction pad, which is inclined with respect to thefriction surface.

Alternatively, the front surface of the friction member is inclined withrespect to the friction surface of the disc rotor where the backingplate has a constant thickness in the direction of movement of thefriction pad while the friction member has a thickness whichcontinuously changes in the above direction of movement. According tothis arrangement, the front surface of the friction member is inclinedwith respect to the back surface of the backing plate, and is recognizedas an inclined surface of the friction pad but is not inclined withrespect to the friction surface of the disc rotor. In this case, too,the back surface of the backing plate which contacts the pressing memberfunctions as the slant surface inclined with respect to the frictionsurface. Thus, the backing plate has the slant surface inclined withrespect to the friction surface of the disc rotor, irrespective ofwhether the friction member or the backing plate has the continuouslychanging thickness.

(22) An electrically operated braking system according to the feature(19) or (20), wherein the self-servo mechanism includes the friction padwhich has a slant surface for contact with the pressing member, theslant surface having an inclination with respect to the frictionsurface, an angle of the inclination of the slant surface changing in adirection in which the friction pad is moved with the disc rotor due tothe friction force therebetween.

In the wedge type self-servo mechanism, the above-indicated slantsurface of the friction pad has a constant angle of inclination withrespect to the friction surface of the disc rotor over the entire lengthof the friction pad in the above-indicated direction of movementthereof. The friction pad can be comparatively easily moved with thedisc rotor when the slant surface has a comparatively small angle ofinclination with respect to the friction surface. That is, the forcethat should be overcome for the friction pad to move with the disc rotoris relatively small when the angle of inclination of the slant surfaceis relatively small. It is also noted that the rate of increase of thefriction force between the friction pad and the disc rotor, namely, therate of increase of the wheel braking force increases with an increasein the inclination angle of the slant surface of the friction pad, inthe wedge type self-servo mechanism. In other words, the rate ofincrease of the self-servo effect increases with an increase in theinclination angle. Therefore, where the inclination angle is constantand relatively large, the movement of the friction pad with the discrotor cannot be easily or smoothly initiated, and the self-servo effectcannot be easily initiated. In addition, the relatively largeinclination angle causes an excessively high rate of increase of theself-servo effect. Thus, the constant inclination angle of the slantsurface of the friction pad makes it difficult to achieve the twoobjectives, that is, easy initiation of the self-servo effect, andprevention of an excessively high rate of increase of the self-servoeffect.

In view of the above, the braking system according to the feature (22)was developed in an effort to achieve the above-indicted to objectives.

In this braking system, the angle of inclination of the slant surface ofthe friction pad with respect to the friction surface of the disc rotorchanges in the direction of the movement of the friction pad with thedisc rotor.

According to the above arrangement, the slant surface may have differentangles of inclination at different portions thereof. These portionsinclude a portion influencing the moment of initiation of the self-servoeffect, a portion substantially influencing the continuation of theself-servo effect, and a portion influencing the prevention of anexcessively high rate of increase of the self-servo effect. Accordingly,the present arrangement makes it possible to achieve the above-indicatedtwo objectives at the same time, namely, easy initiation of theself-servo effect, and prevention of the excessively rapid increase ofthe self-servo effect.

In the braking system according to the feature (22), the entire area ora selected area of the surface of the friction pad for contact with thepressing member is inclined with respect to the friction surface of thedisc rotor. Where the slant surface having a changing angle ofinclination is provided by the entire area of the contact surface of thefriction pad, the slant surface may consist of a single curved surface,a plurality of mutually connected curved surfaces, or a plurality ofmutually connected straight surfaces. For instance, the slant surfaceconsists of a single part-cylindrical surface, a plurality of mutuallyconnected part-cylindrical surfaces, or a plurality of connectedstraight surfaces which are inclined with respect to each other. Wherethe selected area of the contact surface of the friction pad isinclined, the other area is parallel to the friction surface of the discrotor and which is inclined with respect to the slant surface. In thiscase, the selected area may consist of a single straight surface, sincethe non-inclined area which is inclined by 0° with respect to thefriction surface cooperates with this single straight surface to providethe slant surface having two different angles of inclination withrespect to the friction surface of the disc rotor.

The slant surface of the friction pad according to the above feature(22) may be considered to function as the mechanism for mechanicallycontrolling a rate of change of the self-servo effect of the self-servomechanism with a change in the drive force of the electric motor,according to the feature (11) described above.

(23) An electrically operated braking system according to claim (22),wherein said slant surface has a first portion, a second portion whoseangle of inclination with respect to the friction surface of the discrotor is larger than that of the first portion, and a third portionwhose angle of inclination is smaller than that of the second portion,the first, second and third portions being arranged in a directionopposite to the direction of movement of the disc rotor with the discrotor.

In the braking system according to the above feature (23), the pressingmember comes into contact with the first, second and third portions ofthe slant surface as the friction force between the friction pad and thedisc rotor increases. The first portion is formed to permit theinitiation of the self-servo effect. That is, the angle of inclinationof the first portion is made smaller than that of the second portion, tofacilitate the initiation of the movement of the friction pad with thedisc rotor, to permit the initiation of the self-servo effect. Further,the second portion whose angle of inclination is larger than that of thefirst portion assures a sufficient degree of the self-servo effect, andthe third portion whose angle of inclination is smaller than that of thesecond portion prevents an excessively high rate of increase of theself-servo effect.

The angle of inclination of the first portion may be zero, and the angleof inclination of the third portion may be equal to that of the firstportion or may be zero.

(24) An electrically operated braking system according to any one of thefeatures (9), (10) and (19) through (24), wherein the motor-driven discbrake includes a pair of friction pads disposed on opposite sides of thedisc rotor, respectively, one of the friction pads being movable withthe disc rotor due to the friction force therebetween, while the otherof the friction pads being immovable with the disc rotor due to thefriction force, and wherein the pad pressing mechanism includes acaliper extending over a periphery of the disc rotor and movable in thedirection intersecting the friction surface, the caliper comprising areaction portion engageable with the above-indicated other of thefriction pads, and a presser portion for pressing the above-indicatedone of the friction pads against the friction surface, the pad pressingmechanism further including a presser rod which is supported by thepresser portion such that the presser rod is movable by the drive forceof the electric motor in the direction intersecting the frictionsurface, the caliper functioning as the pressing member for theabove-indicated other of the friction pads, while the presser rodfunctioning as the pressing member for the above-indicated one of thefriction pads.

(25) An electrically operated braking system according to any one of thefeatures (9), (10) and (19) through (24), wherein the motor-driven discbrake includes a pair of friction pads disposed on opposite sides of thedisc rotor, respectively, one of the friction pads being movable withthe disc rotor due to the friction force therebetween, while the otherof the friction pads being immovable with the disc rotor due to thefriction force, and wherein the pad pressing mechanism includes acaliper extending over a periphery of the disc rotor and movable in thedirection intersecting the friction surface, the caliper comprising areaction portion engageable with the one of the friction pads, and apresser portion for pressing the above-indicated other of the frictionpads against the friction surface, the pad pressing mechanism furtherincluding a presser rod which is supported by the presser portion suchthat the presser rod is movable by the drive force of the electric motorin the direction intersecting the friction surface, the caliperfunctioning as the pressing member for the above-indicated one of thefriction pads, while the presser rod functioning as the pressing memberfor the other of the friction pads.

In the wedge type self-servo mechanism, the presser rod may be adaptedto contact the above-indicated one friction pad which is movable withthe disc rotor, as in the braking system according to the feature (24).However, the movement of this one friction pad with the disc rotorcauses the same friction pad to slide on the presser rod. This slidingmovement may cause generation of a force which disturbs smooth operationof the presser rod or undesired deformation of the presser rod.

In the braking system according to the above feature (25) wherein thepresser rod is adapted to contact the other friction pad which isimmovable with the disc rotor, there does not arise such a slidingmovement of this other friction pad relative to the presser memberduring activation of the self-servo effect. In this respect, the brakingsystem according to the feature (25) assures normal operation of theself-servo mechanism.

(26) An electrically operated braking system according to claim 1,wherein the electric motor has a non-energized off state, a firstenergized state for forward rotation thereof, and a second energizedstate for reverse rotation thereof, and the pressing member is moved topress the friction pad toward the friction surface of the disc rotorwhen the electric motor is placed in the first energized state, andwherein the motor control device controls the electric motor such thatan actual value of a pressing force by which the friction pad is forcedagainst the friction surface is equal to a desired value, theelectrically operated braking system further comprising insufficientincrease preventing means for preventing a shortage of increase of theactual value of the pressing force by locking the pressing memberagainst a reaction force transferred from the friction pad to thepressing member, when the actual value is required to be increasedduring operation of the self-servo mechanism.

While the pad pressing mechanism is in operation with the electric motorbeing placed in the first energized state, the self-servo effectprovided by the self-servo mechanism is theoretically increased at apredetermined rate, and the actual pressing force of the friction pad istheoretically increased at a predetermined rate. However, the presentinventors found a phenomenon that the self-servo effect and the actualpressing force of the friction pad will not be increased after theself-servo effect and the actual pressing force has been increased togiven values.

The above phenomenon is considered to arise for the following reason:

An increase in the actual pressing force of the friction pad will causean increase in the reaction force which the electric motor receives fromthe friction pad through the pressing member. On the other hand, thedrive torque that can be produced by the electric motor is limited.Accordingly, when the reaction force received from the friction memberbecomes larger than the upper limit of the drive torque of the motor,the motor is operated in the reverse direction by the reaction force ofthe friction pad, and the pressing member is moved in the direction awayfrom the friction pad, so that the self-servo effect and the actualpressing force will no longer be increased. In other words, the motor isoperated in the reverse direction after the self-servo effect hasincreased to a given upper limit and the reaction force of the frictionpad has consequently increased to a given upper limit. Therefore, thereverse operation of the motor does not permit the self-servo effect andthe actual pressing force to be increased after they have exceeded thegiven limits.

The present also found a characteristic of the self-servo mechanism thatthe friction pad can function as a wedge for increasing the actualpressing force while the pressing member is held in the same position,that is, even while the pressing member is not able to continue toadvance the friction pad toward the disc rotor.

In the light of this finding of the characteristic of the self-servomechanism, the braking system according to the above feature (26) wasdeveloped in an effort to solve the problem that the actual pressingforce of the friction pad acting on the disc rotor can no longer beincreased by the self-servo effect of the friction pad.

In this braking system, the insufficient increase preventing means isprovided to prevent the shortage of increase (insufficient rate ofincrease) of the actual pressing force when the actual pressing force isrequired to be increased during operation of the self-servo mechanism.

The motor control device may be adapted to control the electric motor inan open-loop control fashion according to an input command signal, or ina closed-loop control fashion on the basis of the detected actualpressing force as compared with a value represented by the input commandsignal.

The insufficient increase preventing means may be adapted to lock thepressing member by suitable mechanical means or by suitableelectromagnetic or electrical means.

(27) An electrically operated braking system according to the feature(26), wherein the electric motor consists of an ultrasonic motor, andthe motor control device comprises de-energizing means for de-energizingthe ultrasonic motor for thereby enabling the ultrasonic motor toproduce a holding torque for locking the presser member, theinsufficient increase preventing means comprising the de-energizingmeans.

The ultrasonic motor has a known characteristic that the holding torqueproduced when the motor is off or de-energized is larger than the drivetorque produced when it is energized.

The present inventors found that this characteristic of the ultrasonicmotor can be combined with the characteristic of the self-servomechanism that the actual pressing force of the friction pad isincreased by the wedge effect of the friction pad if the pressing membercan be held at the same position, that is, if the motor can bemaintained at the same angular or rotary position.

The braking system according to the feature (27) was developed in thelight of the above-indicated combination of the characteristics of theultrasonic motor and the self-servo mechanism. In this braking system,the motor control device comprises the de-energizing means for turningoff the ultrasonic motor for thereby enabling the motor to produce theholding torque for locking the pressing member, when it is required toincrease the actual pressing force.

In the braking system according to the feature (27), the actual pressingforce which has been increased to the upper limit by the drive torque ofthe ultrasonic motor is further increased by utilizing the holdingtorque of the ultrasonic motor, so that the wheel braking force can beincreased to a value which is larger than the maximum drive torque ofthe ultrasonic motor. Hence, the required size and weight of theultrasonic motor can be reduced, whereby the required size and weight ofthe disc brake can be accordingly reduced.

Since the actual pressing force of the friction pad is increased byholding the ultrasonic motor in its de-energized off state for a givenperiod of time, the required amount of power consumption can be reduced.

The ultrasonic motor may be of travelling-wave (progressive-wave) typeor standing-wave type.

(28) An electrically operated braking system according to the feature(27), wherein the de-energizing means comprises means for de-energizingthe ultrasonic motor when an amount of increase of the actual value ofthe pressing force is smaller than a predetermined first threshold whilethe ultrasonic motor is placed in the first energized state.

In the above braking system, the ultrasonic motor is placed in thede-energized off state when the amount of increase of the actualpressing force becomes smaller than the predetermined first thresholdwhile the motor is in the first energized state. This arrangement iseffective to prevent unnecessary de-energization of the ultrasonicmotor, by permitting the de-energization only when the amount ofincrease of the actual pressing force is detected to be smaller than thethreshold value.

The predetermined first threshold value may be a normal value of theamount of increase of the actual pressing force, which is expected whilethe ultrasonic motor is placed in the first energized state and whilethe drive torque produced by the motor is not smaller than the reactionforce received from the friction pad. The first threshold value may besmaller than this normal value, for instance, zero. Where the firstthreshold value is zero, the ultrasonic motor is de-energized when theactual pressing force begins to be reduced.

(29) An electrically operated braking system according to the feature(28), wherein said insufficient increase preventing means includes (a) asensor for detecting a value relating to the actual pressing force, and(b) increase amount detecting means for obtaining an amount of increaseof the actual pressing force on the basis of an output signal of thesensor.

In the above braking system, the moment of transition of the ultrasonicmotor from the first energized state to the non-energized state isdetermined on the basis of the output signal of the sensor, so that themoment of transition can be controlled with high accuracy in relation tothe actual pressing force of the friction pad.

The sensor may be adapted to directly detect the actual pressing force,or any other parameters which reflect or relate to the actual pressingforce. These parameters include the friction force between the frictionpad and the disc rotor, the wheel braking force, and the decelerationvalue of the vehicle.

(30) An electrically operated braking system according to the feature(29), wherein the motor control device further includes first controlmeans for placing the ultrasonic motor in the first energized stateafter the motor is placed in the de-energized off state by theinsufficient increase preventing means, when the amount of increase ofthe actual pressing force becomes smaller than a predetermined secondthreshold.

The actual pressing force may not be increased as desired even after theultrasonic motor is de-energized with the amount of the actual pressingforce becoming smaller than the first threshold. For the actual pressingforce to be increased by utilizing the holding torque of the ultrasonicmotor, a clearance should not exist between the pressing member and thefriction pad. However, such a clearance may exist for some reason orother. To deal with this case, the ultrasonic motor is brought back tothe first energized state from the de-energized state when the amount ofincrease of the actual pressing force becomes smaller than the secondthreshold after the motor is once placed in the de-energized state bythe insufficient increase preventing means. According to thisarrangement, a clearance if it exists between the friction pad and thepressing member is eliminated by the advancing movement of the pressingmember by the forward operation of the ultrasonic motor placed in thefirst energized state, so that the actual pressing force can beincreased as needed.

Accordingly, the braking system according to the feature (30) assuresadequate operation of the self-servo mechanism.

The second threshold value may be a normal value of the amount ofincrease of the actual pressing force, which is expected while theself-servo mechanism is normally operating. The second threshold valuemay be smaller than this normal value, for example, zero. The secondthreshold value may be equal to or different from the first thresholdvalue.

(31) An electrically operated braking system according to the feature(29), wherein the motor control device further includes second controlmeans for placing the ultrasonic motor in the first energized stateafter the motor is placed in the de-energized off state by theinsufficient increase preventing means, when a predetermined time haspassed after the ultrasonic motor is placed in the de-energized state,irrespective of the amount of increase of the actual pressing forceafter the motor is placed in the de-energized state.

In the above braking system, the ultrasonic motor is brought back to thefirst energized state when the predetermined time has passed after themotor is once placed in the de-energized state with the amount ofincrease of the actual pressing force becoming smaller than the firstthreshold. In this arrangement, too, a clearance if it exists betweenthe friction pad and the pressing member after the motor is oncede-energized is eliminated, so that the self-servo mechanism can operateto achieve the desired self-servo effect.

In the present braking system, the ultrasonic motor is returned to thefirst energized state irrespective of the amount of increase of theactual pressing force after the motor is once de-energized with theamount of increase of the actual pressing force becoming smaller thanthe first threshold. Therefore, the braking system does not require adevice for detecting the amount of increase of the actual pressingforce, which device is required in the system according to the feature(30). Consequently, the braking system is simplified, in particular, inthe software for controlling the ultrasonic motor.

In this braking system, the ultrasonic motor is brought to the firstenergized state even where the motor is required to be kept in thede-energized state for increasing the actual pressing force. In thiscase, the amount of increase of the actual pressing force becomessmaller than the predetermined first threshold, so that the motor isde-energized by the insufficient increase preventing means, for enablingthe motor to produce the holding force for locking the presser member tothereby increase the actual pressing force. Therefore, there arises noproblem in this case.

The predetermined time used by the second control means may be a cycletime or control period of a control routine which is executed by acomputer of the motor control device to control the ultrasonic motor foractivating the pad pressing mechanism. The cycle time or control periodmay be a predetermined constant value or a variable. In this case, themotor control means may be adapted such that if the motor isde-energized by the first control means in a given cycle of execution ofthe control routine as a result of the amount of increase of the actualpressing force being reduced below the predetermined first threshold,the second control means places the motor in the first energized statein the next cycle of execution of the control routine.

(32) An electrically operated braking system according to the feature(27), wherein the de-energizing means comprises means for de-energizingthe ultrasonic motor depending upon whether an operation of theself-servo mechanism has been initiated.

In the above braking system, the ultrasonic motor is de-energizedirrespective of whether the amount of increase of the actual pressingforce is smaller than the predetermined first threshold while the motoris placed in the first energized state.

In this braking system, the motor can be de-energized to produce theholding torque, before the amount of increase of the actual pressingforce is reduced below the first threshold while the motor is in thefirst energized state.

(33) An electrically operated braking system according to the feature(32), wherein the means for de-energizing the ultrasonic motor dependingupon an operation of the self-servo mechanism has been initiatedcomprises a sensor for detecting a value relating to the actual value ofthe pressing force, and self-servo effect monitoring means fordetermining, on the basis of an output signal of the sensor, that theoperation of the self-servo mechanism has been initiated, if each of atleast one predetermined condition is satisfied, the above-indicated atleast one predetermined condition including a condition that the amountof increase of the actual value of the pressing force exceeds apredetermined third threshold while the ultrasonic motor is placed inthe first energized state.

The braking system according to the feature (33) was developed based ona finding that the amount or rate of increase of the actual pressingforce is larger or higher when the self-servo mechanism is in operationthan when the self-servo mechanism is not in operation. In this brakingsystem, the self-servo effect monitoring means determines that theoperation of the self-servo mechanism has been initiated, when all ofthe predetermined condition or conditions is satisfied. Thispredetermined condition includes the condition that the amount ofincrease of the actual pressing force while the motor is in the firstenergized state is larger than the predetermined third threshold.Therefore, the motor is de-energized when this condition is satisfiedtogether with the other predetermined condition or conditions if any.

The third threshold value may be a normal value of the amount ofincrease of the actual pressing force, which is expected while theself-servo mechanism is in operation. The third threshold value may besmaller than this normal value.

(34) An electrically operated braking system according to the feature(33), wherein the above-indicated at least one predetermined conditionfurther includes a condition that the actual pressing force exceeds apredetermined reference value.

The disc brake having the self-servo effect inhibiting mechanismaccording to the feature (5) described above may be designed to initiatethe operation of the self-servo mechanism when the actual pressing forcebecomes larger than a predetermined limit. In this case, theabove-indicated predetermined reference value used according to theabove feature (34) may be equal to or larger than the predeterminedlimit.

The braking system according to the feature (34) permits higher accuracyof detection of the initiation of the self-servo effect, than in thecase where the initiation of the operation of the self-servo mechanismis determined when the amount of increase of the actual pressing forceexceeds the predetermined third threshold while the ultrasonic motor isin the first energized state.

(35) An electrically operated braking system according to the feature(26), wherein the insufficient increase preventing means includes atorque transmission mechanism provided between the electric motor andthe pressing member, so as to permit a torque to be transmitted from themotor to the pressing member and inhibit the torque from beingtransmitted from the pressing member to the motor, for thereby lockingthe pressing member.

In this braking system, the reaction force received by the pressingmember from the friction pad cannot would not be transferred to themotor through the torque transmission mechanism even if the reactionforce became larger than the drive torque of the motor. In thisarrangement wherein the torque transmission mechanism is adapted to lockthe pressing member against the reaction force from the friction pad,the motor is prevented from being operated in the reverse direction bythe reaction force.

The electric motor in the braking system according to the feature (35)may be an ultrasonic motor, a DC motor or any other motor.

(36) An electrically operated braking system according to the feature(35), wherein the pad pressing mechanism includes a motion convertingmechanism comprising a rotatable member which is disposed between theelectric motor and the pressing member and which is rotated by theelectric motor, and a linearly movable member which is linearly movablewith the pressing member, the rotatable and linearly movable membersbeing operatively connected to each other such that a rotary motion ofthe rotatable member is converted into a linear motion of the linearlymovable member, and wherein the torque transmission mechanism isdisposed between the electric motor and the rotatable member, to permitthe torque to be transmitted from the electric motor to the rotatablemember and inhibit the torque from being transmitted from the rotatablemember to the electric motor.

(37) An electrically operated braking system according to the feature(35) or (36), wherein the pad pressing mechanism includes a rotatablemember which is disposed between the electric motor and the pressingmember and which is rotated by the electric motor, and a linearlymovable member which is linearly movable with the pressing member, therotatable member and linearly movable members being operativelyconnected to each other such that a rotary motion of the rotatablemember is converted into a linear motion of the linearly movable member,and wherein the torque transmission mechanism comprises a worm which isdisposed between the electric motor and the rotatable member and whichis rotated by the electric motor and a worm wheel which is rotated bythe worm.

In the braking system according to the feature (37), the torquetransmission mechanism is simple in construction using the worm and theworm wheel.

(38) An electrically operated braking system according to any one of thefeatures (35) through (37), wherein the motor control device includesreverse torque transmission inhibiting means for placing the electricmotor in the non-energized state while the transmission of the torquefrom the rotatable member to the electric motor is inhibited by thetransmission mechanism.

In the braking system according to the feature (38), the electric motoris placed in the non-energized state while the transmission of thetorque from the rotatable member to the electric motor is inhibited bythe torque transmission mechanism, that is, while it is not necessary toplace the electric motor in the energized state.

(39) An electrically operated braking system according to the feature(38), wherein the reverse torque transmission inhibiting means includesself-servo effect inhibition control means for placing the electricmotor in the non-energized state when the operation of the self-servomechanism is initiated.

In the braking system according to the feature (39), the electric motoris de-energized when the self-servo mechanism is in operation. In thisrespect, it is noted that the operation of the self-servo mechanismresults in a high possibility that the transmission of the torque in thereverse direction from the pressing member toward the electric motor isinhibited by the torque transmission mechanism, that is, a highpossibility that the energization of the electric motor is not needed.Therefore, the self-servo effect inhibition control means according tothe feature (39) is effective to prevent unnecessary consumption ofelectric power by the electric motor.

(40) An electrically operated braking system according to the feature(39), wherein the self-servo effect initiation control means comprises(a) a sensor for detecting a value relating to the actual pressing forceof the friction pad, (b) self-servo effect monitoring means fordetermining, on the basis of an output signal of the sensor, that theoperation of the self-servo mechanism has been initiated, if each of atleast one predetermined condition is satisfied, the above-indicated atleast one predetermined condition including a condition that the amountof increase of the actual value of the pressing force exceeds apredetermined second threshold while the ultrasonic motor is placed inthe first energized state.

(41) An electrically operated braking system according to any one of thefeatures (1) through (40), wherein the motor control device comprises(a) at least one information sensor including at least one of anoperation information for obtaining information relating to manipulationof the vehicle by an operator of the vehicle, a vehicle state sensor forobtaining information relating to a running state of the vehicle, and awheel state sensor for obtaining information relating to a state of thewheel of the vehicle, and (b) pressing force determining means fordetermining a desired value of the pressing force of the friction pad onthe basis of an output signal of each of the above-indicated at leastone information sensor, and (c) a controller for controlling theelectric motor such that an actual value of the pressing force coincideswith the desired value determined by the pressing force determiningmeans.

(42) An electrically operated braking system according to any one of thefeatures (1) through (41), wherein the motor control device comprises(a) a primary brake control device for controlling the electric motor tooperate the motor-driven disc brake as a primary brake of the vehicleupon operation of a primary brake operating member, and (b) a parkingbrake control device for controlling the electric motor to operate themotor-driven disc brake as a parking brake of the vehicle upon operationof a parking brake operating member.

(43) An electrically operated braking system according to the feature(42), wherein the primary brake control control device comprises (a) atleast one information sensor including at least one of an operationinformation for obtaining information relating to manipulation of thevehicle by an operator of the vehicle, a vehicle state sensor forobtaining information relating to a running state of the vehicle, and awheel state sensor for obtaining information relating to a state of thewheel of the vehicle, and (b) pressing force determining means fordetermining a desired value of the pressing force of the friction pad onthe basis of an output signal of each of the above-indicated at leastone information sensor, and (c) a primary brake controller forcontrolling the electric motor such that an actual value of the pressingforce coincides with the desired value determined by the pressing forcedetermining means.

(44) An electrically operated braking system according to the feature(42), wherein the parking brake control device comprises (a) a parkingbrake sensor for detecting an operation of the parking brake operatingmember for holding the vehicle stationary, (b) a pressing forcedetermining means for determining a desired value of the pressing forceof the friction pad on the basis of an output signal of the parkingbrake sensor sensor, and (c) a parking brake controller for controllingthe electric motor such that an actual value of the pressing forcecoincides with the desired value determined by the pressing forcedetermining means.

(45) An electrically operated braking system according to any one of thefeatures (1) through (44), further comprising a pressing force sensorfor directly detecting an actual value of the pressing force of thefriction pad generated by the electric motor, and wherein the motorcontrol device includes retracted position control means connected tothe pressing force sensor, for controlling a retracted position of thepressing member which is spaced from the friction pad, when the discbrake is not in operation, the retracted position control meansincluding (a) means for determining, on the basis of an output signal ofthe pressing force sensor, a position at which pressing of the frictionpad by the pressing member is initiated or terminated, (b) means forenergizing the electric motor to retract the pressing member by apredetermined distance from the determined position in a direction awayfrom the friction pad, and (c) de-energizing the electric motor when thepressing member is retracted to the determined position.

(46) An electrically operated braking system comprising:

a motor-driven disc brake including (a) an electric motor as a driveforce for braking a wheel of an automotive vehicle, (b) a disc rotorhaving a friction surface and rotating with the wheel, (c) a frictionpad movable for contact with the friction surface to restrict rotationof the disc rotor, (d) a pad support mechanism for supporting thefriction pad such that the friction pad is movable in a directionintersecting the friction surface of the disc rotor, (e) a pad pressingmechanism comprising the electric motor and a pressing member, theelectric motor producing a drive force for moving the pressing member toforce the friction pad against the friction surface of the disc rotor,and wherein the electric motor has a non-energized off state, a firstenergized state for forward rotation thereof, and a second energizedstate for reverse rotation thereof, the pressing member being moved topress the friction pad toward the friction surface of the disc rotorwhen the electric motor is placed in the first energized state;

a pressing force sensor for directly detecting an actual value of apressing force by which the friction pad is forced against the frictionsurface by the pressing member; and

a motor control device connected to the electric motor and the pressingforce sensor, for controlling the electric motor on the basis of anoutput signal of the pressing force sensor such that the actual value ofthe pressing force represented by the output signal is equal to adesired value,

and wherein the motor control device includes retracted position controlmeans connected to the pressing force sensor, for controlling aretracted position of the pressing member which is spaced from thefriction pad, when the disc brake is not in operation, the retractedposition control means including (i) means for determining, on the basisof the output signal of the pressing force sensor, a position at whichpressing of the friction pad by the pressing member is initiated orterminated, (ii) means for energizing the electric motor to retract thepressing member by a predetermined distance from the determined positionin a direction away from the friction pad, and (iii) de-energizing theelectric motor when the pressing member is retracted to the determinedposition.

In the braking system according to the feature (46), the retractedposition of the pressing member when the disc brake is not in operationis controlled depending upon the actual thickness of the friction pad.This arrangement prevents the friction pad from being located so closeto the disc rotor as to cause dragging of the friction pad with the discrotor when the disc brake is not in operation, and also prevents anexcessively large amount of gap between the friction pad and the discrotor when the disc brake is not in operation. When the gap between thefriction pad and the disc brake is excessively large, the disc brakesuffers from a delay in providing a braking effect.

In the above braking system, the pressing force sensor for detecting theactual value of the pressing force is used to detect the position of thepressing member. Thus, the braking system does not require two sensorsfor detecting the actual pressing force and the position of the pressingmember, respectively.

Further, the retracted position of the pressing member is determineddepending upon its position at which the pressing of the friction pad bythe pressing member is initiated or terminated, that is, depending uponthe position at which the pressing member comes into abutting contactwith the friction pad or is moved apart from the friction pad. Thisarrangement permits accurate determination of the retracted position ofthe pressing member without an influence by a variation in the amount ofelastic deformation of the friction pad, contrary to an arrangementwherein the retracted position is determined depending upon the positionof the pressing member at which the pressing member is fully advanced topress the friction pad against the disc rotor with the maximum pressingforce.

(47) An electrically operated braking system comprising a motor-drivendisc brake according to any one of the features (1) through (46), abraking force sensor for detecting a braking force generated by the discbrake to brake the wheel of the vehicle, and a motor control device forcontrolling the electric motor on the basis of the braking forcedetected by the braking force sensor, such that an actual value of thebraking force is equal to a desired value.

In an electrically operated braking system using an electric motor as adrive source, it is desirable to control the electric motor on the basisof the wheel braking force based on the actual friction force generatedbetween the friction pad and the disc rotor, so that the wheel brakingforce can be accurately controlled irrespective of a variation in thecoefficient of friction between the friction pad and the disc rotor.

In view of the above desirability, the braking system according to thefeature (47) was developed in an effort to control the actual wheelbraking force to the desired value in a feedback fashion, irrespectiveof a variation in the friction coefficient of the friction pad and thedisc rotor.

In the braking system according to the feature (47), the electric motoris controlled while monitoring the actual wheel braking force, so thatthe actual wheel braking force is controlled to coincide with thedesired value, irrespective of the variation in the friction coefficientof the friction pad and the disc rotor.

For example, the braking force sensor is adapted to detect an amount ofstrain or deformation of a selected member of the disc brake, whichamount is relatively accurately proportional to the actual wheel brakingforce.

The motor control device may be adapted to feedback control the electricmotor on the basis of the actual wheel braking force during operation ofthe disc brake, irrespective of whether the self-servo mechanism is inoperation or not. Alternatively, the motor control device may be adaptedto effect the feedback control of the motor only while the self-servomechanism is in operation, or only while the self-servo mechanism is notin operation.

(48) A motor-driven disc brake comprising: a disc rotor having afriction surface and rotating with a wheel of an automotive vehicle; afriction pad movable for contact with the friction surface of the discrotor to restrict rotation of the disc rotor; and a pad pressingmechanism including an electric motor whose drive force is transmittedto the friction pad to force the friction pad against the disc rotor,and wherein the pad pressing mechanism further includes a levercomprising (a) a connecting portion at which the lever is connected to astationary member, pivotally about an axis intersecting an axis ofrotation of the disc rotor, (b) an input portion at which the leverreceives the drive force of the electric motor, and (c) an engagingportion which engages a back surface of the friction pad to transmit thedrive force to the friction pad, wherein the connecting portion, theinput portion and the engaging portion are positioned relative to eachother such that the drive force received from the electric motor isboosted by the lever, so that the boosted drive force is applied to thefriction pad.

In the motor-driven disc brake according to the feature (48), there isprovided a simple boosting mechanism including the lever between theelectric motor and the friction pad. This simple boosting mechanismwhose major part is constituted by the lever permits the disc brake toproduce a wheel braking force which is sufficiently larger than thedrive force produced by the electric motor.

The above disc brake may include the temperature rise restricting meansaccording to the features (17) and (18).

(49) An electrically operated braking system according to any one of thefeatures (1) through (47), wherein the electric motor includes a stator,a rotor and a motor housing in which the stator and the rotor areaccommodated, and the pad pressing mechanism includes: (a) a rotatablemember rotatable about an axis thereof by the electric motor; (b) alinearly movable member disposed rearwardly of the pressing member suchthat the linearly movable member is movable in the directionintersecting the friction surface of the disc rotor; (c) a motionconverting mechanism for converting a rotary motion of the rotatablemember into a linear motion of the linearly movable member, to move thepressing member for forcing the friction pad against the frictionsurface; (d) a caliper including a portion functioning as the motorhousing, and supporting the linearly movable member such that thelinearly movable member is linearly movable; and (e) a rotary supportmechanism for supporting the rotatable member rotatably relative to thecaliper, the rotary support mechanism enabling the caliper to receive asa thrust load a reaction force from the rotatable member while thefriction pad is forced against the friction surface.

In the braking system according to the above feature (49), the electricmotor may be an ultrasonic motor or a wound-rotor type motor.

The linearly movable member may be adapted to be engageable directlywith the back surface of the friction pad, or adapted to move anothermember (e.g., presser rod as described below) which is engageable withthe back surface of the friction pad.

The caliper may consist of a body portion (which may include a presserportion, a reaction portion and a connecting portion, as describedbelow) and the housing portion which functions as the motor housing.These body and housing portions may be separate parts which are boltedor screwed to each other or otherwise fixed to each other to provide thecaliper. Alternatively, the caliper is a one-piece structure consistingof the body and housing portions which are formed integrally with eachother. The caliper may be a floating or fixed type. In the disc brakeusing the floating caliper, the disc rotor generally has oppositefriction surfaces against which two friction pads are forced by theabove-indicated the pressing member and the caliper, respectively.Described more specifically, the reaction force which one of thefriction pads receives from the disc rotor is transmitted by the caliperto the other friction pad.

(50) An electrically operated braking system according to the feature(49), wherein the rotary support mechanism includes a support structurefor reducing an influence of at least one of a first reaction force anda second reaction force upon at least one of the rotatable member andthe electric motor, the first reaction force being received as an offsetload by the rotatable member from the linearly movable member during anoperation of the motor-driven disc brake, and the second reaction forcebeing received by the caliper from the rotatable member during theoperation of the motor-driven disc brake.

A braking system against which the braking system according to the abovefeature (50) was developed to provide an improvement is disclosed inJP-A-8-284980. In this braking system disclosed in this publication, therotary support mechanism includes one radial bearing and one thrustbearing for supporting the rotatable member rotatably relative to thecaliper. These radial and thrust bearings receive a radial load and athrust load of the rotatable member, respectively. The thrust bearing isdisposed between the rotatable member and the housing portion of thecaliper which functions as the motor housing. Further, the body portionof the caliper which is relatively near the friction pad, and thehousing portion of the caliper are separate parts which are screwed toeach other.

When the braking system of the above-identified publication is inoperation, the disc brake is likely to be influenced by a first reactionforce received as an offset load by the rotatable member from thelinearly movable member, and a second reaction force received by thecaliper from the rotatable member.

Described in detail, a reaction force is transferred from the frictionpad directly to the linearly movable member, or indirectly to thelinearly movable member through the pressing member such as a presserrod. At the same time, a reaction force is transferred from the linearlymovable member to the rotatable member, while a reaction force istransferred from the rotatable member to the caliper.

On the other hand, a mounting bracket is generally provided being fixedto the vehicle body in a cantilever form, and a friction force generatedbetween the disc rotor and the friction pad causes a moment to act onthe mounting bracket, thereby causing displacement of the mountingbracket and resulting displacement of the caliper. It is also noted thatthe caliper is not completely symmetrical with respect to a lineparallel to the direction in which it receives the reaction force fromthe friction pad, so that a pressing force by which the friction pad isforced against the disc rotor causes a moment to act on the mountingbracket, causing elastic deformation of the caliper. In some case, theaccuracy of relative positioning of the friction pad, linearly movablemember, rotatable member and caliper is not sufficiently high.

For the above reasons, the line of action of the reaction force which isreceived by the linearly movable member from the friction pad tends tobe offset, misaligned or inclined with respect to the nominal axis ofthe linearly movable member, whereby the line of action of the reactionforce which is received by the rotatable member from the linearlymovable member is also inclined with respect to the nominal axis ofrotation of the rotatable member. Even though the line of action of thereaction force from the friction pad is not inclined with respect to thenominal axis of the linearly movable member, the line of action of thereaction force from the linearly movable member is inclined with respectto the axial of rotation of the rotatable member.

Thus, the reaction force from the linearly movable member acts on therotatable member as an offset load whose line of action is offset fromor inclined with respect to the nominal axis of the rotatable member.

In the braking system disclosed in the above-identified publication,however, only one radial bearing is provided between the rotatablemember and the caliper, so that an offset load acting on the rotatablemember tends to cause the axis of the rotatable member to be inclinedwith respect to the caliper. The inclination of the axis of therotatable member results in an increase in a resistance to rotation ofthe rotatable member, namely, unstable rotation of the rotatable member.

In the braking system of the above-identified publication, the discbrake uses an ultrasonic motor having a stator fixed to the motorhousing, and a rotor coaxially connected to the rotatable member forrotation therewith. In this arrangement, an inclination of the axis ofrotation of the rotatable member with respect to the caliper (includingthe housing portion) causes an inclination of the axis of the rotor,leading to uneven distribution of contact pressure between the rotor andthe stator in their circumferential direction. Consequently, theinclination of the rotatable member may cause abnormal transmission ofthe oscillation of the stator to the rotor, resulting in significantreduction in the drive torque produced by the ultrasonic motor. Thisproblem is not limited to the ultrasonic motor, and may be encounteredin an electric motor of the type in which the stator and the rotor aredisposed with an air gap left therebetween.

It will be understood from the foregoing explanation that the disc brakedisclosed in the publication JP-A-8-284980 suffers from the problem thatthe rotating resistance of the rotatable member is undesirably increasedby its inclination with respect to the caliper due to the reaction force(first reaction force) received from the linearly movable member. Thisdisc brake also suffers from the problem that the drive torque of theelectric motor is undesirably reduced by the inclination of the rotorwith respect to the stator.

In a motor-driven disc brake, it is generally desired that the frictionforce of the friction pad, namely, the braking force be highlyresponsive to an operation of the electric motor during an operation ofthe disc brake in an anti-lock or traction control fashion or for abruptbrake application to the vehicle. To meet this desire, that is, toimprove the response of the braking force, it is considered to increasethe rigidity of the caliper for minimizing its deformation due to thereaction force acting thereon, as well as to improve the operatingresponse of the electric motor.

In the disc brake of the above-identified publication, however, thereaction force from the rotatable member acts on the motor housingthrough the thrust bearing, and the reaction force from the motorhousing acts on the body portion of the caliper. Further, since the bodyportion and the housing portion of of the caliper are fixed to eachother by screws or bolts by other fastening means, the reaction force istransmitted from the motor housing to the body portion of the caliperthrough the fastening means. Therefore, it is required to improve notonly the rigidity of the body portion of the caliper but also therigidity of the housing portion of the caliper, in order to improve theresponse of the disc brake. To increase the rigidity of the housingportion of the caliper, that is, the rigidity of the motor housing, themotor housing must be made of a steel material with a sufficiently largewall thickness. Accordingly, the size and weight of the motor housingtend to be increased, leading to increased size and weight of the discbrake as a whole. To improve the operating response of the disc brake,it is also required to minimize the amount of elongation of thefastening means for fastening the body and housing portions of thecaliper.

It will be understood from the above description that the braking systemdisclosed in the above-identified publication suffers from a relativelylarge load acting on the motor housing, due to the reaction force(second reaction force) which is received by the body portion of thecaliper through the motor housing from the rotatable member while thedisc brake is in operation. Accordingly, the rigidity of the motorhousing which is a part of the electric motor should be increased.

In the light of the above problem, the braking system according to theabove feature (50) was developed in an effort to improve the rotarysupport mechanism for supporting the rotatable member of themotor-driven disc brake.

In the disc brake of the braking system according to the feature (50),the rotary support mechanism includes the support structure which isadapted to reduce an influence of one or both of the first reactionforce and the second reaction force upon one or both of the rotatablemember and the electric motor. The first and second reaction forces aregenerated during operation of the motor-drive disc brake, such that thefirst reaction force is received as an offset load by the rotatablemember from the linearly movable member, while the second reaction forceis received by the caliper from the rotatable member.

In this braking system, the operating response of the disc brake isimproved even in the presence of the reaction force from the frictionpad, owing to the support structure is effective to reduce the influenceof the reaction force upon the rotatable member and/or the electricmotor.

(51) An electrically operated braking system according to the feature(50), wherein the support structure includes a first structure forrestricting an inclination of the axis of the rotatable member by thefirst reaction force during the operation of the motor-driven discbrake.

In this braking system, an increase in the rotating resistance of therotatable member is prevented by the first structure of the supportstructure which is adapted to restrict the inclination of the rotationaxis of the rotatable member by the first reaction force.

(52) An electrically operated braking system according to the feature(51), wherein the rotatable member is coaxially fixed to the rotor forrotation therewith, and the first structure includes a structure forrestricting the inclination of the axis of the rotatable member tothereby restrict an inclination of an axis of the rotor with respect toan axis of the stator.

In this braking system, the inclination of the rotatable member withrespect to the caliper (including the portion functioning as the motorhousing) is restricted, and the inclination of the rotor with respect tothe stator is restricted.

(53) An electrically operated braking system according to the feature(51) or (52), wherein the first structure includes a plurality of radialbearings for rotatably supporting the rotatable member, the radialbearings being spaced apart from each other in an axial direction of therotatable member and receiving a radial load from the rotatable member.

In the braking system, the inclination of the rotatable member isrestricted by the radial bearings of the first structure, which isrelatively simple in construction.

(54) An electrically operated braking system according to the feature(53), wherein the structure of the first structure includes a pluralityof bearings which rotatably support the rotatable member so as toreceive a radial load from at least the rotatable member and which arespaced apart from each other in the axial direction of the rotatablemember.

(55) An electrically operated braking system according to the feature(54), wherein the rotor is coaxially connected to the rotatable memberfor rotation therewith, and the rotatable member includes two axialportions one of which is located on one side of the rotor which isnearer to the friction pad and the other of which is located on theother side of the rotor which is remote from the friction pad.

(56) An electrically operated braking system according to the feature(55), wherein each of the plurality of bearings is mounted on either oneof the two axial portions of the rotatable member.

(57) An electrically operated braking system according to the feature(55), wherein at least one of the plurality of bearings is mounted onthe above-indicated one of the two axial portions of the rotatablemember, and the rest of the plurality of bearings is mounted on theother axial portion.

(58) An electrically operated braking system according to any one of thefeatures (54) through (57), wherein the plurality of bearings includestwo bearings disposed adjacent to opposite ends of the rotatable member,respectively.

(59) An electrically operated braking system according to any one of thefeatures (54) through (58), wherein the plurality of bearings include atleast one radial bearing which rotatably supports the rotatable memberand which receives the radial load from the rotatable member, and atleast one radial thrust bearing which rotatably supports the rotatablemember and which receives the radial load and the thrust load from therotatable member.

In this braking system, the use of the at least one radial thrustbearing each receiving both the radial and thrust loads makes itpossible to reduce the number of the bearings required.

(60) An electrically operated braking system according to according toany one of the features (50) through (59), wherein the support structureincludes a second structure for inhibiting the second reaction forcefrom being transmitted to the electric motor.

In this braking system, the second reaction force is not transmittedfrom the rotatable member to the electric motor, so that the operatingresponse of the disc brake can be improved without having to increasethe rigidity of the motor housing and the rigidity of the fasteningmeans for connecting the body portion of the caliper and the portion ofthe caliper which functions as the motor housing.

(61) An electrically operated braking system according to the feature(60), wherein the rotatable member has a first surface which faces in anaxial direction of the rotatable member from the friction pad toward therotatable member and which transmits the second reaction force to thecaliper, and the caliper has a second surface formed at a portionthereof between the portion thereof functioning as the motor housing anda portion thereof corresponding to the first surface, the second surfacebeing opposed to the first surface in the axial direction of therotatable member and receiving the second reaction force from the firstsurface, the second structure including the first and second surfacesand a bearing which is interposed between the first and second surfacesand between the rotatable member and the caliper such that the bearingrotatably supports the rotatable member so as to receive at least athrust load from the rotatable member.

In this braking system, the operating response of the disc brake can beimproved with a simple arrangement of the rotatable member and thecaliper.

(62) An electrically operated braking system according to the feature(61), wherein the first surface of the rotatable member faces in abackward direction from the friction pad toward the rotatable member,while the second surface of the caliper faces in a frontward directionfrom the rotatable member toward the friction pad and is opposed to thefirst surface.

(63) An electrically operated braking system according to any one of thefeatures (60) through (62), wherein the caliper includes a front portionwhich is located on one side of the second surface nearer to thefriction pad and which consists of an integrally formed one-piecesection.

In this braking system, the rigidity of the caliper can be easily madehigher than that of the caliper whose front portion consists of two ormore parts bolted or screwed to each other. Accordingly, the operatingresponse of the disc brake can be improved.

(64) An electrically operated braking system according to any one of thefeatures (49) through (63), wherein the electric motor is an ultrasonicmotor including a stator adapted to generate a surface wave uponapplication of an ultrasonic oscillation thereto, and a rotor which isrotated with a friction force between the rotor and the stator.

In this braking system, the ultrasonic motor may be of a travelling-wavetype or a standing-wave type.

(65) An electrically operated braking system according to the feature(50), wherein the support structure includes the first structureaccording to any one of the features (51) through (59), and the secondstructure according to any one of the features (60) through (63).

In this braking system, the support structure is effective to restrictnot only an increase in the rotating resistance of the rotatable memberduring operation of the disc brake, but also a decrease in the drivetorque of the electric motor, and is also effective to improve theoperating response of the disc brake without having to increase therigidity of the motor housing.

BRIEF DESCRIPTION OF DRAWINGS

The above and optional objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings, in which:

FIG. 1 is a view schematically showing an electrically operated brakingsystem constructed according to a first embodiment of this invention,including a plan view of a motor-driven disc brake in the system;

FIG. 2 is a cross sectional view taken along line 2—2 of FIG. 1;

FIG. 3 is an enlarged plan view and an enlarged elevational view incross section of a portion of the disc brake indicated by a circle A inFIG. 1;

FIG. 4 is a block diagram schematically showing an operation of acontroller shown in FIG. 1, for controlling the disc brake;

FIG. 5 is a block diagram indicating various functional means of thecontroller of FIG. 1;

FIG. 6 is a flow chart illustrating a brake control routine executed bya computer of the controller of FIG. 1;

FIG. 7 is a graph indicating a relationship among a brake pedaldepression force f and front and rear wheel braking forces Ff, Fr in thefirst embodiment of FIG. 1;

FIG. 8 is an enlarged plan view and an enlarged elevational view incross section of the portion of a disc brake according to onemodification of the first embodiment, which portion corresponds to thatof FIG. 3;

FIG. 9 is a plan view schematically showing a portion of a motor-drivedisc brake of an electrically operated braking system constructedaccording to a second embodiment of the present invention;

FIG. 10 is a plan view partly in cross section of a motor-driven discbrake of an electrically operated braking system according to a thirdembodiment of the invention;

FIG. 11 is a view in cross section taken in a plane extending throughand parallel to an outer pad of the disc brake of FIG. 10;

FIG. 12 is a view in cross section taken in a plane extending throughand parallel to an inner pad of the disc brake of FIG. 10;

FIG. 13 is a view schematically showing an electrically operated brakingsystem constructed according to a fourth embodiment of this invention,including a plan view of a motor-driven disc brake in the system;

FIG. 14 is an enlarged perspective view of a cooling device in thebraking system of FIG. 13;

FIG. 15 is a block diagram schematically showing an arrangement of anelectrically operated braking system according to a fifth embodiment ofthe invention;

FIG. 16 is a view schematically showing an electrically operated brakingsystem according to a sixth embodiment of the invention, including aplan view of a motor-driven disc brake in the system;

FIG. 17 is a view schematically showing an electrically operated brakingsystem according to a seventh embodiment of the invention, including aplan view of a motor-driven disc brake in the system;

FIG. 18 is a view schematically showing an electrically operated brakingsystem according to an eighth embodiment of the invention, including aplan view of a motor-driven disc brake in the system;

FIG. 19 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to aninth embodiment of the invention;

FIG. 20 is a plan view showing the motor-driven disc brake of FIG. 19;

FIG. 21 is an enlarged plan view of an elastic control mechanism in thedisc brake of FIG. 20;

FIG. 22 is a plan view showing a modification of the elastic controlmechanism of FIG. 21;

FIG. 23 is a front elevational view of the motor-drive disc brake ofFIG. 19;

FIG. 24 is a top plan view of a piezoelectric body used in themotor-driven disc brake of FIG. 19, showing an arrangement ofelectrodes;

FIG. 25 is a bottom plan view of the piezoelectric body of FIG. 24,shown in an arrangement of electrodes;

FIG. 26 is a block diagram illustrating an electrical arrangement of thebraking system of FIG. 19;

FIG. 27 is a block diagram showing details of a motor driver circuit,and connection of the motor driver circuit to a DC power source and aultrasonic motor;

FIG. 28 is a flow chart illustrating a brake control routine executedaccording to a program stored in a ROM of a computer of a primary brakecontroller shown in FIG. 26;

FIG. 29 is a flow chart illustrating a pad pressing control routineimplemented in step S15 of the routine of FIG. 28;

FIG. 30 is a graph indicating an example of control of a pad pressingforce according to the brake control routine of FIG. 28;

FIG. 31 is a graph indicating another example of control of the padpressing force according to the brake control routine of FIG. 28;

FIG. 32 is a flow chart illustrating a presser rod initial positioncontrol routine implemented in step S18 of the routine of FIG. 28;

FIG. 33 is a flow chart illustrating a parking brake control routineexecuted according to a program stored in a ROM of a computer of aparking brake controller shown in FIG. 26;

FIG. 34 is a flow chart illustrating a pad pressing control routineexecuted according to a program stored in a ROM of a primary brakecontroller in an electrically operated braking system constructedaccording to a tenth embodiment of this invention;

FIG. 35 is a flow chart illustrating a pad pressing control routineexecuted according to a program stored in a ROM of a computer of aprimary brake controller in an electrically operated braking systemaccording to an eleventh embodiment of the invention;

FIG. 36 is a time chart indicating a motor drive signal generated as aresult of execution of the routine of FIG. 35;

FIG. 37 is a flow chart illustrating a pad pressing control routine in abrake control routine executed according to a program stored in a ROM ofa computer of a primary brake controller in an electrically operatedbraking system according to a twelfth embodiment of the invention;

FIG. 38 is a flow chart illustrating a self-servo effect monitoringroutine implemented in step S151 of the routine of FIG. 37;

FIG. 39 is a graph indicating an example of control of a pad pressingforce according to the pad pressing control routine of FIG. 37;

FIG. 40 is a graph indicating a change in ultrasonic motor drivefrequency, which is effected by a frequency tracer in a motor drivercircuit in an electrically operated braking system according to athirteenth embodiment of the invention;

FIG. 41 is a flow chart illustrating a pad pressing control routine in abrake control routine executed according to a program stored in a ROM ofa computer of a primary brake controller in the braking system of FIG.40;

FIG. 42 is a flow chart illustrating a force decreasing control routineimplemented in step S160 of the routine of FIG. 41;

FIG. 43 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to afourteenth embodiment of the invention;

FIG. 44 is a block diagram indicating an electrical arrangement of thebraking system of FIG. 43;

FIG. 45 is a flow chart illustrating a pad pressing control routine in abrake control routine executed according to a program stored in a ROM ofa computer of a primary brake controller shown in FIG. 44;

FIG. 46 is a plan view of an elastic control mechanism of a motor-drivendisc brake in an electrically operated braking system according to afifteenth embodiment of the invention;

FIG. 47 is a graph indicating elastic characteristics of the elasticcontrol mechanism of FIG. 46;

FIG. 48 is a plan view showing one modification of the elastic controlmechanism of FIG. 46;

FIG. 49 is a plan view showing another modification of the elasticcontrol mechanism of FIG. 46;

FIG. 50 is a fragmentary plan view in cross section of a motor-drivendisc brake in an electrically operated braking system according to asixteenth embodiment of the invention;

FIG. 51 is a plan view partly in cross section showing one modificationof the sixteenth embodiment of FIG. 50;

FIG. 52 is a plan view of a motor-driven disc brake in an electricallyoperated braking system according to a seventeenth embodiment of theinvention;

FIG. 53 is a front elevational view of the disc brake of FIG. 52;

FIG. 54 is an enlarged front elevational view partly in cross sectionshowing an end portion 544 b shown in FIG. 53;

FIG. 55 is a plan view for explaining one modification of a connectionbetween end portion 543 a and portion 538 a shown in FIG. 52;

FIG. 56 is a plan view showing one modification of elastic member 542shown in FIG. 52;

FIG. 57 is a plan view showing another modification of the elasticmember 542;

FIG. 58 is a front elevational view partly in cross section of amotor-driven disc brake in an electrically operated braking systemaccording to an eighteenth embodiment of the invention;

FIG. 59 is an enlarged side elevational view of outer pad 14 a in thedisc brake of FIG. 58;

FIG. 60 is an enlarged side elevational view of outer pad of amotor-driven disc brake in an electrically operated braking systemaccording to a nineteenth embodiment of the invention;

FIG. 61 is an enlarged side elevational view of outer pad of amotor-driven disc brake in an electrically operated braking systemaccording to a twentieth embodiment of the invention;

FIG. 62 is an enlarged side elevational view of a motor-driven discbrake in an electrically operated braking system according to atwenty-first embodiment of the invention;

FIG. 63 is a plan view partly in cross section of a motor-driven discbrake in an electrically operated braking system according to atwenty-second embodiment of the invention;

FIG. 64 is a plan view partly in cross section of a motor-driven discbrake in an electrically operated braking system according to atwenty-third embodiment of the invention;

FIG. 65 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-fourth embodiment of the invention;

FIG. 66 is a plan view of the disc brake of FIG. 65;

FIG. 67 is a block diagram indicating an electrical arrangement of thebraking system of FIG. 65;

FIG. 68 is a flow chart illustrating a brake control routine executedaccording to a program stored in a ROM of a computer of a controllershown in FIG. 67;

FIG. 69 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-fifth embodiment of the invention;

FIG. 70 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-sixth embodiment of the invention;

FIG. 71 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-seventh embodiment of the invention;

FIG. 72 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-eighth embodiment of the invention; and

FIG. 73 is a side elevational view in cross section of a motor-drivendisc brake in an electrically operated braking system according to atwenty-ninth embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring first to FIG. 1, there is shown an electrically operatedbraking system constructed according to a first embodiment of thepresent invention, for use on a 4-wheel automotive vehicle. The brakingsystem has four motor-driven disc brakes for braking respective fourwheels of the vehicle. In FIG. 1, only one of these four motor-drivendisc brakes is shown generally at 10.

The disc brake 10 has a disc rotor 11 functioning as a rotary memberwhich is rotated with the wheel to be braked. The disc rotor 11 hasopposite friction surfaces 12, 12, while the disc brake 10 includes apair of friction pads 14, 14 disposed opposite to the respectivefriction surfaces 12, 12 of the rotor 11. Each of these two frictionpads 14 has a friction member 18, and a backing plate 20 which is fixedto the back surface of the friction member 18 and which is made of asteel material.

Adjacent to the disc rotor 11, there is disposed a mounting member inthe form of an anchor member 26 secured to the body of the vehicle, suchthat the anchor member 26 extends over the periphery of the disc rotor11 in the direction of an axis of the disc rotor 11, namely, in thevertical direction as seen in FIG. 1. To the opposite ends of the anchormember 26, there are pivotally connected a pair of levers 30, 30, suchthat the disc rotor 11 and the two friction pads 14, 14 are interposedbetween the two levers 30.

Each lever 30 has a front end portion (right end portion as seen inFIG. 1) at which the lever 30 is connected to the anchor member 26pivotally about a first axis L1 as also indicated in FIG. 2, which isperpendicular to the axis of the disc rotor 11. Described in detail, theanchor member 26 has a pair of connecting portions 32, 32 formed at itsopposite ends such that the connecting portions 32 are located on theopposite sides of the disc rotor 11 so as to face the respectivefriction surfaces 12, 12. As shown in FIG. 2, each of the connectingportions 32 is formed as a pair of extensions 36 extending in therearward direction of the vehicle, with a space 34 left therebetween.The front end portion of the lever 30 has a first connecting portion 40which is accommodated in the space 34 with small amounts of clearancesto the extensions 36, 36. A connecting member in the form of a screw 42extends through the extensions 36, 36 and the first connecting portion40, so that the lever 30 is pivotable about the axis L1 of screw 42relative to the connecting portion 32.

Each lever 30 has an engaging portion 44 at an intermediate partthereof, as shown in FIG. 1. The engaging portion 44 engages the backsurface of the backing plate 20 of the corresponding friction pad 14.The lever 30 also has a first bearing portion 50 at its front endportion. As shown in FIG. 2, this first bearing portion 50 engages anengaging cutout 46 formed in the front end face of the backing plate 20which faces in the forward direction of the vehicle. The first bearingportion 50 has an end face which faces in the rearward direction of thevehicle and which engages the bottom surface of the cutout 46.

As shown in FIG. 1, the two levers 30 are connected to each other by apair of links 54, 54 at their intermediate parts between the engagingportion 44 and the rear end portions (left end portions as seen in FIG.1). The two links 54 are connected at their ends to each other by a pin56 pivotally about a second axis L2 (axis of the pin 56) which isparallel to the first axis L1 (axis of the screw 42). The two links 54are further connected, at their other ends remote from the pin 56, tosecond connecting portions 62 of the levers 30 by respective pins 60,pivotally about third axes L3 (axes of the pins 60) which are parallelto the second axis L2. Each of the two links 54 has a second bearingportion 68 which engages an engaging cutout 66 formed in the rear endface of the backing plate 20 which faces in the rearward direction ofthe vehicle, as shown in FIG. 2. The second bearing portion 68 has anend face which faces in the forward direction of the vehicle and whichengages the bottom surface of the cutout 66. The function of these links54 will be described.

Thus, each friction pad 14 is supported at its front end by engagementwith the first bearing portion 50 of the lever 30, and at its rear endby engagement with the second bearing portion 68 of the link 54, suchthat the friction pad 14 is movable toward and away from thecorresponding friction surface 12 of the disc rotor 11.

A first pressing device 70 is associated with the rear end portions ofthe levers 30, 30. The first pressing device 70 includes an electricmotor in the form of a ultrasonic motor 72, and a motion convertingmechanism 76 for converting a rotary motion of a rotary shaft 74 of theultrasonic motor 72 into a linear motion.

The ultrasonic motor 72 has a stator and a rotor. In operation, thestator produces a surface wave upon application of a ultrasonicvibration thereto, and the rotor is rotated with a friction force actingbetween the stator and the rotor, as well known in the art. The rotor isforced against the stator by suitable biasing means, so that a suitableamount of friction force acts between the stator and the rotor. Evenwhen no voltage is applied to the ultrasonic motor 72, a certain amountof friction force exists between the stator and the rotor. Theultrasonic motor 72 is attached to an input portion 77 provided at therear end portion of one of the two levers 30, that is, to the inputportion 77 of the lower lever 30 as seen in FIG. 1, while the motionconverting mechanism 76 is connected to an input portion 78 provided atthe rear end portion of the other lever 30 (upper lever 30).

As shown in FIG. 3, the motion converting mechanism 76 is a screwmechanism including an externally threaded member 80 rotating with therotary shaft 74 of the ultrasonic motor 72, and an internally threadedmember 82 which engages the externally threaded member 80. Theinternally threaded member 82 takes the form of a ball 90 accommodatedin the rear end portion of the upper lever 30. The ball 90 is slidablyfitted within a receptacle 92 formed in the rear end portion of theupper lever 30, and cooperates with the receptacle 92 to provide a balljoint 84. When the externally threaded member 80 is moved relative tothe ball joint 84 by rotation of the rotary shaft 74, the rear endportions of the two levers 30, 30 are moved relative to each other, andthe levers 30, 30 are pivoted relative to each other about the axes L1.The receptacle 92 is partially defined by a spherical surface 92 aformed in the upper lever 30, more precisely, in a part of the rearportion of the upper lever 30, which is on the side of the other orlower lever 30. The spherical surface 92 a terminates into an accessopening 92 b which is open on the other side of the upper lever 30 andthrough which the ball 90 is moved into the receptacle 92. The upperlever 30 has removal preventing means in the form of a C-ring 94 forpreventing the ball 90 from being removed out of the receptacle 92. Theopening 92 b is closed by a cover 96, which cooperates with the outersurface of the ball 90 and the inner surface of the receptacle 92 todefine a space, which is filled with a grease, for assuring a smoothsliding movement of the ball 90 relative to the inner surface of thereceptacle 92.

In the present first embodiment of the electrically operated brakingsystem, a rotary motion of the rotary shaft 74 of the ultrasonic motor72 in one of opposite directions will cause the rear end portions of thepair of levers 30 to be moved toward each other, so that the engagingportions 44, 44 of the two levers 30 are moved toward each other,whereby the friction pads 14 are forced against the opposite frictionsurfaces 12 of the disc rotor 11. As a result, the disc rotor 11 isbraked with friction forces generated between the friction pads 14 andthe friction surfaces 12 of the disc rotor 11, whereby the wheel of theautomotive vehicle is braked by the disc brake 10.

When the ultrasonic motor 72 is operated in the reverse direction in theabove-indicated condition, the two levers 30 are pivoted relative toeach other such that their rear end portions are moved away from eachother, whereby the engaging portions 44 are moved away from each other.As a result, the friction pads 14 are moved away from the frictionsurfaces 12 of the disc rotor 11, and the braking force acting on thewheel is reduced or zeroed.

When the wheel is braked by activation of the disc brake 10 duringrunning of the vehicle in the forward direction, friction forces act onthe friction pads 14 in the forward direction of the vehicle, and aretransmitted to the first bearing portions 50 of the levers 30, so thatmoments act on the levers 30 so as to pivot the levers 30 about thefirst axes L1 in the opposite directions for moving the engagingportions 44 toward each other and the friction surfaces 12 of the discrotor 11. When the wheel is braked during running of the vehicle in therearward direction, friction forces act on the friction pads 14 in therearward direction of the vehicle, and are transmitted to the secondbearing portions 68 of the links 54, so that moments act on the links 54so as to pivot the links 54 about the second axis L2 (about the pin 56)in the opposite directions for moving the pins 60 toward each other,whereby moments acts on the levers 30 so as to pivot the levers 30 aboutthe first axes L1 in the opposite direction for moving the engagingportions 44 toward each other and the friction surfaces 12 of the discrotor 11.

Thus, the moment acts on each lever 30 in the direction for moving theengaging portion 44 toward the disc rotor 11 upon braking of the wheelduring running of the vehicle, irrespective of the vehicle runningdirection (either forward or rearward direction). As a result, thefriction pad 14 is forced against the friction surface 12 of the discrotor 11 by the engaging portion 44, by a second pressing force based onthe friction force between the friction pad 14 and the friction surface12. Thus, a first pressing force based on the drive force of theultrasonic motor 72 is boosted. This boosting of the pressing force isreferred to as “self-servo effect”.

In the present first embodiment, the pair of levers 30, 30 and the pairof links 54, 54 cooperate to constitute a second pressing device 98, andthe pair of levers 30, 30 also function as a part of the first pressingdevice 70.

It will be understood from the above explanation of the presentembodiment that the two first bearing portions 50 of the pair of levers30, 30 and the two second bearing portions 68 of the pair of links 54,54 cooperate to constitute a pad support mechanism for supporting thefriction pads 14. It will also be understood that the levers 30 functionas a pressing member for pressing the friction pads 14 against the discrotor 11, and the ultrasonic motor 72, levers 30 and motion convertingmechanism 76 cooperate to constitute a pad pressing mechanism forpressing the friction pads 14 against the disc rotor 11, while thesecond pressing device 98 functions as a self-servo mechanism forboosting the force generated by the first pressing device 70.

The ultrasonic motor 72 is controlled by a motor control device in theform of a controller 100, which is adapted to control the ultrasonicmotor 72 of the disc brake 10 for each wheel such that a detected actualbraking force F acting on the wheel coincides with a desired value F*which corresponds to a brake operating amount f. This control of theultrasonic motor 72 by the controller 100 is effected in a feedbackfashion as indicated in FIG. 4.

For the controller 100 to effect this feedback control of the ultrasonicmotor 72, there is provided a brake operating amount sensor in the formof a depression force sensor 102 connected to the controller 100, asshown in FIG. 1. This depression force sensor 102 is adapted to detect adepression force f acting on a brake pedal 104 as a brake operatingmember when the brake pedal 104 is depressed by the vehicle operator. Anoutput signal of the depression force sensor 102 represents thedepression force f. Also connected to the controller 100 is a powersupply 106 for energizing the ultrasonic motor 72. The power supply 106may be a battery provided on the vehicle. To the controller 100, thereare also connected braking force sensors 110 for detecting the actualbraking forces F acting the respective wheels of the vehicle. Forinstance, each of these braking force sensors 110 uses a strain gageattached to s suitable member (e.g., lever 30) of the disc brake 10which is subject to a strain proportional to the braking force F actingon the wheel.

Referring to the block diagram of FIG. 5, there will be describedfunctional means of the controller 100. The controller 100 incorporates(a) brake operating amount calculating means 120, (b) desired brakingforce calculating means 122, (c) actual braking force calculating means124, (d) drive signal calculating means 126, and (e) drive signalapplying means 128. The brake operating amount calculating means 120 isadapted to calculate, as the operating amount of the brake pedal 104,the depression force f on the basis of the output signal of thedepression force sensor 102. The desired braking force calculating means122 is adapted to calculate the desired braking force F* (desired valueF* of the braking force F) on the basis of the depression force fcalculated as the brake operating amount. The actual braking forcecalculating means 124 is adapted to calculate the actual braking force Facting on each vehicle wheel, on the basis of the output signals of thebraking force sensors 110. The drive signal calculating means 126 isadapted to calculate a drive signal for energizing the ultrasonic motor72, on the basis of an error ΔF between the calculated actual anddesired braking forces F and F*, so that the actual braking force Fcoincides with the desired braking force F*. The drive signal applyingmeans 128 is adapted to apply the calculated drive signal to theultrasonic motor 72 of the disc brake 10 for wheel wheel.

The controller 100 is principally constituted by a computer including acentral processing unit (CPU), a read-only memory (ROM) and arandom-access memory (RAM). The CPU is adapted to execute a brakecontrol routine illustrated in the flow chart of FIG. 6, according to aprogram stored in the ROM functioning as a data storage medium, whileutilizing a temporary data storage function of the RAM.

The brake control routine of FIG. 6 is started when an ignition switchof an engine of the vehicle is turned on, and repeatedly executed with apredetermined cycle time. The routine is initiated with step S1 in whichthe brake pedal depression force f is calculated on the basis of theoutput signal of the depression force sensor 102. Step S1 is followed bystep S2 to calculate the desired braking force F* for each wheel on thebasis of the calculated depression force f, and according to apredetermined relationship between the depression force f and desiredtotal front and rear braking forces Ff*, Fr*. The desired total frontbraking force Ff* is a desired sum of the braking forces of the frontright and left wheels, while the desired total rear braking force Fr* isa desired sum of the braking forces of the rear right and left wheels.The above-indicated relationship, an example of which is indicated inthe graph of FIG. 7, is represented by a table, data map or functionalequation stored in the ROM of the controller 100. Initially, the desiredtotal front braking force Ff* is obtained on the basis of the depressionforce f and according to the predetermined relationship, and a halfvalue of the obtained desired total front braking force Ff* is obtainedas a desired front right braking force Ffr* and a desired front leftbraking force Ffl*. Then, the desired total rear braking force Fr* isobtained on the basis of the depression force f and according to thepredetermined relationship, and a half value of the obtained desiredtotal rear braking force Fr* is obtained as a desired rear right brakingforce Frr* and a desired rear left braking force Frl*.

Then, the control flow goes to step S3 to calculate the actual brakingforce Ffl, Ffr, Frl, Frr acting on each wheel, on the basis of theoutput signal of the corresponding braking force sensor 110. Step S3 isfollowed by step S4 to calculate the drive signal for energizing theultrasonic motor 72, on the basis of the error ΔF between the calculatedactual and desired braking forces F and F*, so that the drive signalpermits the ultrasonic motor 72 to be energized so that the actualbraking force F acting on each wheel coincides with the desired valueF*. For instance, the drive signal to be applied to the ultrasonic motor72 may be calculated according to the following PID equation:

K·[ΔF=(t/Ti)·ΣΔF+(Td/t)·ΔΔF]

where,

K: proportional coefficient (constant)

ΔF: error=F*−F

t: sampling time (cycle time of the routine of FIG. 6)

Ti: integration time (constant)

Td: differentiation time (constant)

ΔΔF: time derivative of error ΔF

Then, the control flow goes to step S5 in which the calculated drivesignal is applied to the ultrasonic motor 72 of the motor-driven discbrake 10 for each wheel. Thus, one cycle of execution of the brakecontrol routine of FIG. 6 is completed.

The present embodiment of the electrically operated braking systemincluding the motor-driven disc brake 10 and the controller 100 has thefollowing advantages:

Since it is not necessary to use a working fluid for braking the wheel,it is not necessary to use hydraulic or pneumatic components such as amaster cylinder, a brake booster, brake tubes and hoses, a proportioningvalve, solenoid-operated valves, a pump and a reservoir. Accordingly,the present braking system can be assembled with improved efficiency,and can be made compact with reduced size and weight, leading to areduced weight of the vehicle and an increased space for passengers.Further, it is not required to replace the working fluid and effect airbreathing of the hydraulic system, leading to increased ease ofmaintenance of the braking system. In addition, the present brakingsystem permits free setting of a relationship between the operatingforce acting on the brake operating member (brake pedal 104) and theoperating stroke of the brake operating member. In this respect, it isnoted that if a master cylinder was used for operating the disc brake10, the diameter of a piston of the master cylinder would determine therelationship between the brake operating force and stroke and providesubstantially no freedom in setting this relationship.

The present embodiment has a further advantage. That is, the disc brake10 has a comparatively small number of components used in a powertransmission path from the ultrasonic motor 72 to the friction pads 14,since the levers 30 constitute a major portion of the power transmissionpath. Accordingly, the power transmission path is simple inconstruction, and permits a high response of the actual braking force Fto a change in the desired value F*. Moreover, the self-servo effectprovides a further improvement in the control response of the brakingforce. In addition, the disc brake 10 may be suitably used as a frictionbrake in an electric motor vehicle or a hybrid vehicle. Namely, thefriction brake 10 may be adequately controlled even when the disc brake10 is operated during regenerative braking of the wheels by amotor-generator of the electric vehicle. Described more specifically,the braking force acting on a wheel upon activation of the frictionbrake by operation of the brake pedal during the regenerative brakingconsists of a first braking force component generated by the frictionbrake and a second braking force component generated by the regenerativebrake. Since the first braking force component can be controlled asneeded by controlling the ultrasonic motor 72, the total braking forceacting on the wheel can be adequately controlled to a value accuratelycorresponding to the operating amount of the brake pedal, even thoughthe second braking force component varies with the rotating speed of thewheel. Thus, the present electrically operated braking system includingthe disc brake 10 activated by the ultrasonic motor 72 facilitatescoordination of the friction brake with the regenerative brake in anelectric or hybrid vehicle.

Various changes and modifications may be made in the present embodiment.While the C-ring 94 is used in the motion converting mechanism 76 ofFIG. 3 to prevent the removal of the ball 90 out of the receptacle 92,the C-ring 94 may be replaced by a retainer ring 130 as shown in FIG. 8,which is shaped to have an increased surface area in sliding contactwith the ball 90 and reduce a spacing between the ball 90 and the cover96. The retainer ring 130 may be made of a synthetic resin, such asnylon having a high self-lubricating property, for reducing the slidingresistance of the ball 90. The retainer ring 130 may be slidably fittedin the receptacle 92 and forced against the ball 90 under a biasingforce generated by the cover 96 made of an elastic material, so that theretainer ring 130 and the elastic cover 96 cooperate with the lever 30to elastically hold the ball 90 within the receptacle 92, whilepreventing oscillation of the ball 90 within the receptacle 92.

A second embodiment of the present invention will then be described. Anelectrically operated braking system according to this second embodimentis identical with that of the first embodiment, except for a self-servomechanism. Therefore, only the self-servo mechanism of the secondembodiment will be described in detail.

In the first embodiment, the self-servo effect is provided such that thefriction forces acting on the friction pads 14 in the rotating directionof the disc rotor 11 during activation of the disc brake 10 are returnedto the friction pads 14 through the levers 30. In the second embodiment,on the other hand, the self-servo effect is provided owing to a wedgeeffect of each friction pad 14 whose backing plates 20 has a slantsurface 144 engaging a slant surface 142 of a drive member 140 which isdriven by the force G generated by the ultrasonic motor 72, as shown inFIG. 9. The drive member 140 may be driven directly by the ultrasonicmotor 72 or through a suitable motion converting mechanism.

In this second embodiment, the drive member 140 constitutes a pressingmember for pressing the friction pad 14 against the disc rotor 11, andthe ultrasonic motor 72 and the drive member 140 cooperate to provide apad pressing mechanism for pressing the friction pad 14 against the discrotor 11. Further, the friction pad 14 having the slant surface 144formed on the backing plate 20 to enable the friction pad 14 to functionas a wedge provides a self-servo mechanism.

A third embodiment of this invention will be described by reference toFIG. 10. Like the second embodiment, this third embodiment utilizes awedge effect of the friction pads 14 to provide the self-servo effect.The same reference signs as used in the first embodiment will be used inthe third embodiment to identify the functionally correspondingcomponents, which will not be described to avoid redundancy.

In the third embodiment of FIG. 10, the electrically operated brakingsystem includes a disc brake 150 for each wheel of the 4-wheel vehicle,and the controller 100, depression force sensor 102 and power supply 106which are commonly used for the four wheels. Each disc brake 150 has abraking force sensor 110 whose output signal is fed to the controller100.

The disc brake 150 includes a mounting member in the form of a mountingbracket 152 fixed to the vehicle body. The mounting bracket 152 includesportions for supporting the two friction pads 14 a, 14 b on the oppositesides of the disc rotor 11, such that the friction pads 14 a, 14 b aremovable in the axial direction of the disc rotor 11. The mountingbracket 152 further includes portions for receiving friction forces fromthe friction pads 14 a, 14 b in frictional contact with the frictionsurfaces 12 of the disc rotor 11, in the rotating direction of the discrotor 11.

Referring to FIG. 11, there is shown the outer pad 14 a as supported bythe mounting bracket 152. The outer pad 14 a is the friction pad 14 a(upper friction pad as seen in FIG. 10) located on the outer side of thevehicle. In FIG. 11, an arrow X indicates the forward rotating directionof the disc rotor 11. The outer pad 14 a has a front end face 156 facingin the forward rotating direction X, and a rear end face 158 facing inthe reverse rotating direction opposite to the direction X. The outerpad 14 a includes an engaging protrusion 160 and an engaging protrusion162 which protrude from the front and rear end faces 156, 158, 35respectively. The mounting bracket 152 has two engaging cutouts 164, 166formed so as to extend in the axial direction of the disc rotor 11. Theengaging protrusions 160, 162 of the outer pad 14 a engage therespective engaging cutouts 164, 166 of the mounting bracket 152 suchthat the protrusions 160, 162 are slidable relative to the mountingbracket 152 in the axial direction of the disc rotor 11, and movablewithin the engaging cutouts 164, 166 in a direction perpendicular to theaxial direction of the rotor 11. The outer pad 14 a is normally held ina radially outer position by a biasing force of a spring 168, which actson the outer pad 14 a in the radially outer direction of the disc rotor10. Thus, otherwise possible rattling movement of the outer pad 14 awithin the mounting bracket 152 is prevented. Further, the outer pad 14a is supported by the mounting bracket 152 so as to substantiallyprevent “dragging” of the outer pad 14 a along with the disc rotor 11,that is, substantially prevent a movement of the outer pad 14 a due tofrictional contact with the disc rotor 11.

Referring next to FIG. 12, there is shown the inner pad 14 b assupported by the mounting bracket 152. The inner pad 14 a is thefriction pad 14 b (lower friction pad as seen in FIG. 10) located on theinner side of the vehicle. Like the outer pad 14 a, the inner pad 14 bhas a front end face 170 facing in the forward rotating direction X, anda rear end face 172 facing in the reverse rotating direction opposite tothe direction X. The inner pad 14 b includes an engaging protrusion 174and an engaging protrusion 176 which protrude from the front and rearend faces 170, 172, respectively. The mounting bracket 152 has twoengaging cutouts 178, 180 formed so as to extend in the axial directionof the disc rotor 11. The engaging protrusions 174, 176 of the inner pad14 b engage the respective engaging cutouts 178, 180 of the mountingbracket 152 such that the protrusions 174, 176 are slidable relative tothe mounting bracket 152 in the axial direction of the disc rotor 11,and movable within the engaging cutouts 178, 180 in the directionperpendicular to the axial direction of the rotor 11. The inner pad 14 bis normally held in a radially outer position by a biasing force of aspring 182, which acts on the inner pad 14 b in the radially outerdirection of the disc rotor 10. Thus, otherwise possible rattlingmovement of the inner pad 14 b within the mounting bracket 152 isprevented.

Unlike the outer pad 14 a, the inner pad 14 b is supported by themounting bracket 152 so as to positively allow the dragging movement ofthe inner pad 14 b with the disc brake 11, that is, a movement of theinner pad 14 b due to frictional contact with the disc rotor 11. In FIG.12, an arrow Y indicates a direction in which the inner pad 14 b isdragged with the disc rotor 11. To allow the dragging of the inner pad14 b, a comparatively large gap is left in the direction Y between thefront end face 170 and the opposite surface of the mounting bracket 152.Further, the bottom of the engaging cutout 178 engaging the frontengaging protrusion 174 of the inner pad 14 b is movable in thedirection Y so that the depth of the cutout 178 is variable.

Described in detail, the bottom of the engaging cutout 178 is defined bya movable member 186 which is forced against the end face of theengaging protrusion 174 under a biasing action of a spring 184.Normally, the movable member 186 is held by the spring 184 in its fullyretracted position which is determined by abutting contact with a stop188. When the friction force acting between the friction surface 12 ofthe disc rotor 11 and the inner pad 14 b exceeds a predeterminedthreshold, the inner pad 14 b (engaging protrusion 174) is allowed to bemoved with the movable member 186 by the friction force against thebiasing action of the spring 184. While the friction force acting on theinner pad 14 b is smaller than the threshold, the inner pad 14 b isprevented by the biasing force of the spring 184 from being moved in thedirection Y. Thus, the inner pad 14 b is allowed to be dragged with thedisc rotor 11 only after the friction force between the inner pad 14 band the disc rotor 11 exceeds the predetermined threshold. To limit thedistance of dragging movement of the inner pad 14 b due to itsfrictional contact with the disc rotor 11, the movable member 186 isprovided with a stop 190, which inhibits the movement of the inner pad14 b when the distance of the movement reaches a predetermined upperlimit. Thus, the stop 190 limits the distance of the dragging movementof the inner pad 14 b, thereby limiting the self-servo effect of theinner pad 14 b.

Referring back to FIG. 10, the disc brake 150 further includes a caliper202 which is movable in the axial direction of the disc rotor 11 but isnot movable in the rotating direction of the disc rotor 11.

As indicated by two-dot chain lines in FIGS. 11 and 12, the caliper 202slidably engages a plurality of pins 204 which are attached to thevehicle body so as to extend in the axial direction of the disc rotor11. The caliper 202 slidably movably supported by the pins 204 extendsover the periphery of the disc rotor 11, as indicated in FIG. 10, andhas two portions located opposite to the backing plates 20 of the outerand inner friction pads 14 a, 14 b. Described more particularly, thecaliper 202 includes a reaction portion 206 disposed adjacent to theouter surface of the backing plate 20 of the outer pad 14 a, a presserportion 208 disposed adjacent to the outer surface of the backing plate20 of the inner pad 14 b, and a connecting or intermediate portion 210connecting the reaction and presser portions 206, 208.

The presser portion 208 carries a motor in the form of a ultrasonicmotor 212 coaxially connected to a presser rod 216 through a motionconverting mechanism in the form of a ballscrew mechanism 214. Thepresser rod 216 is supported by the presser portion 208 such thatpresser rod 216 is not rotatable about its axis but is axially movablerelative to the presser portion 208. A rotary motion of a rotary shaft218 of the ultrasonic motor 212 is converted by the ballscrew mechanism214 into a linear motion of the presser rod 216, whereby the inner pad14 b is forced by the presser rod 216 against the corresponding frictionsurface 12 of the disc rotor 11. At the same time, a reaction force istransferred from the inner pad 14 b to the outer pad 14 a through thecaliper 202, so that the outer pad 14 a is forced by the reactionportion 206 against the other friction surface of the disc rotor 11.

In the present third embodiment, the caliper 202 functions as a pressingmember, and cooperates with the ultrasonic motor 212, ballscrewmechanism 214 and presser rod 216 to constitute a pad presser mechanismfor pressing the friction pads 14 against the disc rotor 11.

While the thickness of the backing plate 20 of the outer pad 14 a isconstant, the thickness of the backing plate 20 of the inner pad 14 bcontinuously decreases in the direction Y in which the inner pad 14 b ismoved due to dragging with the disc rotor 11. In other words, thebacking plate 20 of the inner pad 14 b has a slant exposed surface 220,which is inclined with respect to the friction surfaces 12 of the discrotor 11. The presser rod 216 is held in contact the slant surface 220at its front end face such that the presser rod 216 and the slantsurface 220 are movable relative to each other when the inner pad 14 bis moved in the direction Y. In this arrangement, the backing plate 20of the inner pad 14 functions as a wedge between the disc rotor 11 andthe presser rod 216 when the inner pad 14 b is dragged with the discrotor 11 in the direction Y, whereby the inner pad 14 b provides aself-servo effect. In the present embodiment, the axis of the presserrod 216 is perpendicular to the slant surface 220 of the backing plate20 of the inner pad 14 b.

To assure smooth relative movement of the presser rod 216 and the innerpad 14 b, the presser rod 216 has a plurality of balls 222 held on itsend face such that the balls 222 are arranged in an equally spaced-apartrelation along a circle concentric with the circumference of the presserrod. The balls 222 are partially exposed on the end face of the presserrod 216 and can be rolled in contact with the slant surface 220. Theballs 222 may be replaced by rollers. Thus, the balls 222 function as athrust bearing as indicated at 224, which is interposed between thebacking plate 20 of the inner pad 14 b and the presser rod 216, forreducing a friction resistance between between the inner pad 14 b andthe presser rod 216. In the present embodiment, the thrust bearing 224provides means for reducing the friction resistance between the innerpad 14 b and the end face of the presser rod 216. This frictionresistance reducing means may be provided by using a suitable materialfor at least the end portion of the presser rod 216 which contacts theslant surface 222 of the backing plate 20 of the inner pad 14 b. Thismaterial should have higher degrees of wear resistance, corrosionresistance than the metal used for the backing plate 20, and a highdegree of slidability with respect to the slant surface 222. Forinstance, the material may be selected from among silicon nitride,silicon carbide, highly wear-resistant ceramics, self-lubricatingpolyamide resins, and self-lubricating, highly wear-resistantfluoro-resins suitable for improving rust resistance of the presser rod216.

In the present embodiment, the balls 222 are made of a material having ahigher degree of thermal insulation property than a metallic material,for example, silicon nitride, silicon carbide, and ceramics havingcomparatively high thermal insulation property. The balls 222 made ofsuch a thermally insulating material interposed between the ultrasonicmotor 212 and the inner pad 14 b are effective to minimize an amount oftransfer of heat generated due to friction between the inner pad 14 band the disc rotor 11, to the ultrasonic motor 212 through the powertransmission path, thereby restricting a rise of the temperature of theultrasonic motor 212. Thus, the balls 222 having high thermal insulationproperty function as means for restricting the temperature rise of theultrasonic motor 212, and means for restricting the heat transfer fromthe inner pad 14 b to the ultrasonic motor 212. These temperature riserestricting means and the heat transfer restricting means may beprovided by forming the presser rod 216 of a material having a higherdegree of thermal insulation property than a metallic material.

An operation of the present braking system will then be described.

When the ultrasonic motor 212 is energized as a result of an operationof the brake pedal 104 by the vehicle operator, the presser rod 216 ismoved from its retracted position to its advanced position, so that theinner pad 14 b is forced against the disc rotor 11. Consequently, afriction force is generated between the inner pad 14 b and the discrotor 11. At the same time, the outer pad 14 a is forced against thedisc rotor 11, and a friction force is generated between the outer pad14 a and the disc rotor 11. Thus, the vehicle wheel is braked by thedisc brake 150.

While the friction force of the inner pad 14 b is not larger than athreshold determined by a set load of the spring 184, the movement ofthe inner pad 14 b in the direction Y due to dragging movement with thedisc rotor 11 is prevented by the spring 184, and the self-servo effectof the inner pad 14 b is not exhibited. Therefore, the vehicle wheel isbraked by only the drive force generated by the ultrasonic motor 212while the friction force of the inner pad 14 b is relatively small, forexample, immediately after the depression of the brake pedal 104 orduring an initial period of operation of the disc brake 150 with arelatively small amount of depression force acting on the brake pedal104.

When the friction force of the inner pad 14 b becomes larger than thethreshold due to an increase in the depression force acting on the brakepedal 104, the friction force overcomes the biasing action of the spring184, whereby the inner pad 14 b is allowed to be moved together with themovable member 186 against the biasing force of the spring 184. As aresult, the force acting between the friction surface 12 of the discrotor 11 and the presser rod 216 is increased owing to a wedge effect ofthe backing plate 20 having the slant surface 220, whereby the inner pad14 b is forced onto the disc rotor 11 with the increased force in theaxial direction of the rotor 11.

In other words, the inner pad 14 b functions as a wedge between the discrotor 11 and the presser rod 216, exhibiting a self-servo effect forboosting the braking force based on the drive force of the ultrasonicmotor 212, when the friction force acting on the inner pad 14 b isrelatively large with the brake pedal 104 depressed with a depressionforce large enough to achieve the intended deceleration of the vehicle,for instance, about 0.3-0.6 G.

When the force by which the inner pad 14 b is forced against the discrotor 11 is increased by the self-servo effect or function of the innerpad 14 b, the force acting on the pressure rod 216 in the axialdirection is accordingly increased, so that the torque of the rotaryshaft 218 of the ultrasonic motor 212 is accordingly increased. In thelight of this fact, the ballscrew mechanism 214 is arranged to have arelatively small reverse efficiency, and the ultrasonic motor 212 isadapted to have a relatively high torque holding ability.

When the stop 190 of the movable member 186 comes into contact with themounting bracket 152 as a result of a further movement of the inner pad14 b in the direction Y with a further increase in the friction force, afurther movement of the inner pad 14 b in the direction Y is inhibitedby the stop 190, whereby an increase in the self-servo effect isinhibited. The stop 190 prevents an excessive force between the backingplate 20 of the inner pad 14 b and the presser rod 216, which wouldcause local permanent deflection or deformation of the backing plate 20and resulting permanent dragging of the inner pad 14 b due to itsfailure to return to the predetermined non-operated position when thepresser rod 216 is returned from the advanced or operated position tothe non-operated position upon releasing of the brake pedal 104.

In the present third embodiment, the inner pad 14 b is supported by themounting bracket 152 such that the inner pad 14 b can be moved in thedirection Y due to frictional contact with the disc rotor 11, and thebacking plate 20 of the inner pad 14 b is provided with the slantsurface 220, so that the inner pad 14 b functions as a wedge. Thus, theinner pad 14 b having the slant surface 220 on the backing plate 20serves as a self-servo mechanism.

The present third embodiment is further adapted such that the front gapbetween the front end face 170 of the inner pad 14 b and the oppositesurface of the mounting bracket 152 while the disc brake 150 is in thenon-operated position is larger than the rear gap between the rear endface 172 and the opposite surface of the mounting bracket 152. Theamount of the rear gap is determined to be enough for the inner pad 14 bto be slidably movable relative to the mounting bracket 152 in the axialdirection of the disc rotor 11. In other words, the amount of the frontgap is determined to be larger than this amount of the rear gap, inorder to allow the inner pad 14 b to be dragged with the disc rotor 11due to frictional contact of the inner pad 14 b with the disc rotor 11.Thus, the front gap cooperates with the movable member 190 and thespring 184 to constitute a mechanism for allowing the inner pad 14 b tobe dragged with the disc rotor 11. It is also noted that the spring 184functions as an elastic member for inhibiting the self-servo mechanismfrom providing the self-servo effect while the friction force betweenthe inner pad 14 b and the disc rotor 11 is smaller than thepredetermined threshold. Further, the stop 190 serves as a mechanism forinhibiting the self-servo mechanism from providing an excessiveself-servo effect.

It will be understood from the above explanation of the third embodimentthat the self-servo mechanism for boosting the drive force of theultrasonic motor 212 to obtain a sufficient wheel braking force is notprovided throughout a braking operation, but is provided only after thefriction force between the inner pad 14 b and the disc rotor 11 exceedsthe predetermined threshold, or only during an initial period of thebraking operation. The wheel braking force tends to be unstable if theself-servo mechanism is operated while the friction force between theinner pad 14 b and the disc rotor 11 is relatively small. Thus, thepresent embodiment permits a sufficient increase in the wheel brakingforce when needed, without instability of the wheel braking force due tothe self-servo effect. The present embodiment enjoys the advantage ofthe self-servo mechanism, with substantial elimination of itsdisadvantage.

There will next be described a fourth embodiment of this invention,which is similar to the third embodiment in various aspect. The samereference signs as used in the third embodiment will be used in thefourth embodiment to identify the functionally corresponding elements.

A disc brake 230 constructed according to the fourth embodiment is shownin FIG. 13. The disc brake 230 is characterized by the provision of acooling device 232 for positively cooling the ultrasonic motor 212 tothereby restrict a temperature rise of the ultrasonic motor 212. Thetemperature rise of the ultrasonic motor 212 may be caused by not onlyheat generation due to the friction between the friction pads 14 a, 14 band the disc rotor 11, but also heat generation from the coil of themotor 212. Therefore, the temperature rise of the motor 212 cannot besufficiently restricted by merely restricting the transfer of thefriction heat to the motor 212. The present embodiment was developed inthe light of this fact.

The cooling device 232 is of a water cooling type provided with a waterjacket 234 enclosing the housing of the ultrasonic motor 212. The waterjacket 234 has a passage system 236 through which water or othersuitable liquid is circulated by a pump 238, which is driven by a pumpmotor 240. The pump 238 is connected to a reservoir 242. The pump motor240 is controlled by the controller 100 to suitably turn on and off thepump 238. The cooling device 232 is shown in FIG. 14 wherein the waterjacket 234 is shown enlargement. In the present fourth embodiment, thecooling device 232 functions as the temperature rise restricting means.

In the fourth embodiment, the ultrasonic motor 212 is positively cooledby the cooling device 232 so as to effectively restrict a rise of thetemperature of the motor 212, for thereby avoiding abnormalities of themotor 212 which may be caused by its temperature rise.

The cooling device 232 of the water cooling type used in the presentembodiment may be replaced by an air cooling type of cooling deviceincluding a cooling fan for blowing air toward the ultrasonic motor 212,and an electric motor for driving the cooling fan. The air cooling typecooling device may be easily designed for cooling a comparatively largearea of the disc brake 230, and is preferably designed to cool not onlythe ultrasonic motor 212 but also the friction pads 14 a, 14 b and thedisc rotor 11, which generate heat during operation of the disc brake230.

Referring to FIG. 15, there will be described a fifth embodiment of theinvention which is similar to the fourth embodiment. With the samereference signs as used in the fourth embodiment being used in FIG. 15,only a difference of the fifth embodiment from the fourth embodimentwill be described in detail.

In the fourth embodiment, the ultrasonic motor 212 for braking the discrotor 11 is cooled by the cooling device 232 which uses the electricmotor 240. In the present fifth embodiment, a cooling device 252 forcooling the ultrasonic motor 212 uses this ultrasonic motor 212 as adrive source. Described more specifically, the ultrasonic motor 212 isoperatively connected through a power transmission control device 250selectively to the friction pads 14 a, 14 b and the cooling device 252.This cooling device 252 includes a pump or fan which is driven by theultrasonic motor 212 through the power transmission control device 250,for producing a stream of a liquid or gas toward the ultrasonic motor212. The power transmission control device 250 is adapted to transmit adrive force of the ultrasonic motor 212 to the friction pads 14 a, 14 bwhen the disc brake 130 is required to be activated for braking the discrotor 11, and to transmit the drive force to the cooling device 252during at least a portion of the period in which the activation of thedisc brake 130 is not required.

Usually, the brake pedal 104 is not depressed continuously for a longtime, but is alternately depressed and released with the depressingaction taking place intermittently. The temperature of the ultrasonicmotor 212 rises during depression of the brake pedal 104, and the driveforce of the ultrasonic motor 212 can be used for other purposes whilethe brake pedal 104 is in the released state. Therefore, the ultrasonicmotor 212 may be controlled to operate for braking the wheel only whenthe brake pedal 104 is depressed, and for cooling the ultrasonic motor212 so as to effectively restrict a rise of the temperature of theultrasonic motor 212 during intermittent braking operations.

In the light of the above finding, the power transmission control device250 is adapted to transmit the drive force of the ultrasonic motor 212to the friction pads 14 a, 14 b when the brake pedal 104 is depressed(when the output signal of the depression force sensor 102 indicates thedepression of the brake pedal 104), and to the cooling device 252 whenthe brake pedal 104 is not depressed (when the output signal of thesensor 102 does not indicates the depression of the brake pedal 104). Inthe present fifth embodiment, the ultrasonic motor 212, powertransmission control device 250 and cooling device 252 cooperate toconstitute the temperature rise restricting means.

The cooling device provided in the fourth and fifth embodiments of FIGS.13-15, which uses a motor as the drive source, may be used for cooling adisc brake which does not have the self-servo mechanism.

In all of the embodiments described above, the self-servo action of thefriction pads for converting the friction force of the friction padsinto the pressing force acting on the disc rotor through the frictionpads takes place in the power transmission system through which thedrive force of the ultrasonic motor is transmitted to the friction pads.On the other hand, the dragging of the friction pads or pad along withthe disc rotor 11 is controlled (permitted or inhibited) at the locationat which the friction force is received. Thus, the self-servo action andthe control of the dragging of the friction pads or pad take place atthe different locations within the disc brake. This arrangement permitssimpler and more reliable mechanisms for achieving the self-servo effectand for permitting and inhibiting the dragging of the friction pads orpad, than the arrangement in which the self-servo action and thedragging control take place at one location.

Referring next to FIGS. 16-18, sixth, seventh and eighth embodiments ofthis invention will be described. These embodiments are similar to thethird embodiment.

In the third embodiment, the braking force sensor 110 is adapted todetect, as the braking force, the force which the mounting bracket 152receives from the inner pad 14 b. In the sixth embodiment of FIG. 16, abraking force sensor 260 is interposed between two separate portions ofthe presser rod 216, to detect a force which the presser rod 216receives from the inner pad 14 b. This force relates to the brakingforce for braking the disc rotor 11. In the seventh embodiment of FIG.17, a braking force sensor 262 is interposed between the caliper 202 andthe rear portion of the ultrasonic motor 212 remote from the presser rod216. This sensor 262 detects a force which the ultrasonic motor 212receives from the inner pad 14 b through the presser rod 216. This forcealso relates to the braking force. In the eighth embodiment of FIG. 18,a braking force sensor 264 is provided on the front end of the presserrod 216. The sensor 264 has a generally part-spherical convex surface incontact with the slant back surface 220 of the backing plate 20 of theinner pad 14 b. The sensor 264 detects a force which the presser rod 216receives from the inner pad 14 b. This force also relates to the brakingforce.

In the present embodiment of FIG. 15, the ultrasonic motor 212 forbraking the disc rotor 11 is effectively cooled for improved operatingstability of the motor 212, and the motor 240 used in the fourthembodiment of FIGS. 13 and 14 can be eliminated, leading to reduced costof manufacture of the disc brake 230.

Referring to FIGS. 19-33, there will be described an electricallyoperated braking system constructed according to a ninth embodiment ofthe present invention, for use on a 4-wheel automotive vehicle. Thebraking system has four motor-driven disc brakes for braking respectivefour wheels of the vehicle. In FIG. 19, only one of these fourmotor-driven disc brakes is shown generally at 310.

The motor-driven disc brake 310 has a disc rotor 312 functioning as arotary member which is rotated with the wheel to be braked. The discrotor 312 has opposite friction surfaces 314, while the disc brake 310includes a pair of friction pads 320 a, 320 b disposed opposite to therespective friction surfaces 314 of the disc rotor 312. Each of thesetwo friction pads 320 has a friction member 322, and a backing plate 324which is fixed to the back surface of the friction member 322 and whichis made of a steel material.

The disc brake 310 includes a pad support mechanism 326, a self-servomechanism 327, and a pad presser mechanism 328.

The pad support mechanism 326 will be described first.

As shown in FIG. 20, the disc brake 310 is provided with a mountingbracket 330 which is fixed to the body of the vehicle so as to extendover the periphery of the disc rotor 312. The mounting bracket 330includes (a) portions which are located on the opposite sides of thedisc rotor 312 and which support the respective friction pads 320 a, 320b such that the friction pads 320 are movable in a directionintersecting the friction surfaces 314, and (b) portions functioning asa bearing member, which portions receive friction forces generated dueto frictional contacts of the friction pads 320 with the frictionsurfaces 314 of the disc rotor 312. In FIG. 20, “X” represents adirection of rotation of the disc rotor 312 during forward running ofthe vehicle, while “Y” represents a direction in which each of thefriction pads 320 is movable relative to the friction surfaces 314. Themounting bracket 330 is fixed to the vehicle body such that the upperportion of the mounting bracket 330 as seen in FIG. 20 is located on thefront side of the vehicle while the right and left portions of themounting bracket 330 as seen in FIG. 20 are located on the outer andinner sides of the vehicle as viewed in the lateral or transversedirection of the vehicle. Therefore, the friction pad 320 a on the rightside of the vehicle is referred to as an outer pad while the frictionpad 320 b on the left side is referred to as an inner pad.

Then, the self-servo mechanism 327 will be described.

The self-servo mechanism 327 is adapted to enable the inner pad 320 b tofunction as a wedge which provides a self-servo effect. To this end, theinner pad 320 b is supported by the mounting bracket 330 such that theinner pad 320 b is positively allowed to be dragged along with the discrotor 312 due to frictional contact of the inner pad 320 b with the discrotor 312. The structure of the mounting bracket 330 for supporting theinner pad 320 b in this manner is similar to that in the thirdembodiment of FIGS. 10-12. In FIG. 20, “Z” represents a direction inwhich the inner pad 320 b is dragged with the disc rotor 312 during theforward running of the vehicle. The inner pad 320 b is wedge-shaped withthe thickness of the friction member 322 continuously decreasing in thedragging direction “Z”, namely, in the direction from the rear sidetoward the front side of the vehicle. Thus, the friction member 322 ofthe inner pad 320 b has a slant surface 334 which is inclined withrespect to the opposite surfaces of the backing plate 324 and which isopposed to the friction surface 314 of the disc rotor 312. With theslant surface 334 held in contact with the friction surface 314, theback surface of the backing plate 324 remote from the friction member322 is inclined with respect to the friction surface 314. Thus, thebacking plate 324 is inclined with respect to the friction surface 314.For a presser rod (which will be described) to engage the backing plate324 such that the axis of the presser rod is perpendicular to the backsurface of the backing plate 324, the mounting bracket 330 is fixed tothe vehicle body such that a reference line L1 of the mounting bracket330 is inclined with respect to an axis L2 of rotation of the disc rotor312 so that the left portion of the mounting bracket 330 as seen in FIG.20 is displaced toward the front portion of the vehicle. The referenceline L1 is a straight line which passes the centers of the friction pads320 a, 320 b and is parallel to the direction Y in which the pads 320are movable. The reference line L1 is also parallel to the direction inwhich a caliper 336 engageable with the backing plates 324 of thefriction pads 320 is slidably movable relative to the mounting bracket330 to which the caliper 336 is slidably attached through pins.

The outer pad 320 a is not intended to provide a self-servo effect. Inthis sense, the outer pad 320 a need not be wedge-shaped. However, theouter pad 320 a is also wedge-shaped following the angle of inclinationof a caliper 336 whose direction of movement is parallel to thereference line L1 of the mounting bracket 330 which is inclined withrespect to the rotation axis L2 of the disc rotor 312 by the angle ofinclination of the backing plate 324 of the inner pad 320 b with respectto the friction surfaces 314 of the disc rotor 312. Unlike the frictionmember 322 of the inner pad 320 b,the friction member 322 of the outerpad 320 a has a thickness which continuously increases in the draggingdirection Z of the inner pad 320 b or in the rotating direction X of thedisc rotor 312. The wedge shape of the outer pad 320 a permits itsfriction member 322 to contact the friction surface 314 of the discrotor 312 without a gap or clearance therebetween over the entire areaof the friction surface 314.

As described above, the mounting bracket 330 supports the inner pad 320b so as to positively allow the inner pad 320 b to be moved or draggedwith the disc rotor 312 due to the frictional contact therebetween.However, the mounting bracket 330 supports the outer pad 320 b so as tosubstantially inhibit the outer pad 320 a from being moved with the discrotor 312.

As in the third embodiment, the inner pad 320 b is not always allowed tobe moved with the disc rotor 312. Namely, the inner pad 320 b issupported such that the movement of the inner pad 320 b with the discrotor 312 is permitted only after the friction force acting on the innerpad 320 b exceeds a predetermined threshold. Described morespecifically, the inner pad 320 b is associated with the mountingbracket 330 via an elastic control mechanism 340 as shown in FIG. 21.The elastic control mechanism 340 has an elastic member which receives aload from the inner pad 320 b.The elastic member is not elasticallydeformed until the received load is smaller than the predeterminedthreshold, so that the inner pad 320 b is inhibited from being movedrelative to the mounting bracket 330 in the dragging direction Z, thatis, inhibited from being moved with the disc rotor 312, until the loadacting on the elastic member is smaller than the threshold. After theload exceeds the threshold, the elastic member of the elastic controlmechanism 340 is elastically deformed, allowing the inner pad 320 b tobe moved relative to the mounting bracket 330 and moved or dragged withthe disc rotor 312.

The elastic control mechanism 340 will be described in detail byreference to FIG. 21. The mechanism 340 includes (a) a U-shaped elasticmember 342 having a pair of arms, and (b) an adjusting mechanism 344 forchanging an initial amount of elastic deformation of the elastic member342, to thereby adjust a pre-load acting on the elastic member 342. Thispre-load is equal to the above-indicated predetermined threshold abovewhich the inner pad 320 b is permitted to be moved in the draggingdirection Z against the biasing action of the elastic member 342. Theelastic member 342 is positioned such that the pair of arms extend inthe lateral or transverse direction of the vehicle. One of the arms issecured to the mounting bracket 330 while the other arm is fixed to theinner pad 320 b.The adjusting mechanism 344 includes an adjusting boltwhich extends in a direction substantially parallel to the draggingdirection Z and which connects the two arms of the elastic member 342 soas to permit movements of the two arms toward each other and inhibitmovements of the two arms away from each other. The adjusting boltpermits adjustment of the spacing distance between the two arms tothereby permit adjustment of the pre-load acting on the elastic member342.

The elastic control mechanism 340 may be replaced by another elasticcontrol mechanism 350 shown in FIG. 22. This elastic control mechanism350 includes (a) an elastic mechanism 352 consisting of a plurality ofconed disc springs coaxially superposed on each other, (b) atransmission mechanism 354 for transmit an elastic force of the elasticmechanism 352 to the inner pad 320 b,and (c) an adjusting mechanism 356for adjusting a pre-load acting on the elastic mechanism 352. Thetransmission mechanism 352 is a U-shaped elastic member having a pair ofarms, which is similar to the elastic member 342 of the elastic controlmechanism 340 of FIG. 21. The U-shaped elastic member of thetransmission mechanism 352 is positioned such that the arms extend inthe transverse direction of the vehicle. One of the arms is secured tothe mounting bracket 330 while the other arm is fixed to the inner pad320 b.In the present elastic control mechanism 350, the elasticmechanism 352 is provided to produce an elastic force acting on theinner pad 320 b,while the transmission mechanism 354 is provided totransmit this elastic force to the inner pad 320 b. Accordingly, thetransmission mechanism 354 need not be large-sized as compared with theU-shaped elastic member 342 of FIG. 21. The adjusting mechanism 352includes an adjusting bolt similar to that of the adjusting mechanism344 of FIG. 21, for adjusting a pre-load of the elastic mechanism 352 bychanging an initial amount of elastic deformation of the coned discsprings.

In the present ninth embodiment, the thickness of the friction member322 of the inner pad 320 b continuously decreases in the rotatingdirection X while the thickness of the backing plate 324 of the innerpad 320 b is constant in the rotating direction X, as indicated in FIG.20. Thus, the slant surface 344 is provided on the friction member 322.However, the ninth embodiment may be modified such that the thickness ofthe friction member 322 of the inner pad 320 b is constant while thethickness of the backing plate 324 continuously decreases in therotating direction X, so that the slant surface is provided on thebacking plate 324. This modification is also possible with respect tothe outer pad 320 a.

In this embodiment, the predetermined threshold of the friction force ofthe inner pad 320 b,or the pre-load of the elastic control mechanism340, 350 is equal to the friction force which is generated between thedisc rotor 312 and the inner pad 320 b when the deceleration of thevehicle achieved by activation of the disc brake 310 is about 0.5-0.6 G.When the deceleration of the vehicle is lower than this threshold ofabout 0.5-0.6 G with the brake pedal being operated in an ordinary ornormal manner, the elastic control mechanism inhibits the dragging ofthe inner pad 320 b with the disc rotor 312 to thereby inhibit aself-servo effect of the inner pad 320 b.When the vehicle decelerationexceeds the threshold with the brake pedal being abruptly depressed by arelatively large amount, the elastic control mechanism allows the innerpad 320 b to be dragged with the disc rotor 312, permitting the innerpad to achieve the self-servo effect.

It will be understood from the above explanation that the elasticcontrol mechanism 340, 350 constitutes a mechanism for inhibiting theinner pad 320 b from providing the self-servo effect under apredetermined condition, namely, while the friction force of the innerpad 320 b is smaller than a predetermined threshold.

The pad presser mechanism 328 will then be explained.

As indicated above, the disc brake 310 includes the caliper 336 shown inFIGS. 19 and 20. As shown in FIG. 19, the caliper 336 has a body portion358, and a bracket 360 which is bolted to the body portion. 358. Thebracket 360 is located on the inner side of the body portion 358 as seenin the transverse direction of the vehicle, for supporting an ultrasonicmotor which will be described. The caliper 336 also has a pair of arms361 which extend in the longitudinal direction of the vehicle as shownin FIG. 20 and which are bolted to the body portion 358 as shown in FIG.23. The pair of arms 361 are also bolted to respective portions of thebracket 360 as also shown in FIG. 23. It is noted that FIG. 23 is a viewof the caliper 336 taken in the left direction as seen in FIG. 20. InFIG. 23, the body portion 358 and the arms 361 are indicated by solidlines, while the bracket 360 is indicated y two-dot chain line.

While the caliper 336 consists of the separate members, namely, bodyportion 358, bracket 360 and arms 361 which are bolted together, thecaliper may be an integral one-piece structure.

As shown in FIGS. 19 and 20, the caliper 336 is supported at the bodyportion 358 by the mounting bracket 330 such that the caliper 336 isslidably movable in the direction Y in which the friction pads 320 aremovably supported by the mounting bracket 330. The two arms 361 areconnected at their end portions to respective two pins 362 which extendin the direction Y. These two pins 362 engage the mounting bracket 330such that the pins 362 are slidable in the direction Y. Thus, thecaliper 336 are slidably supported by the mounting bracket 330, at thebody portion 358 and through the two pins 362.

The body portion 358 of the caliper 336 consists of a presser portion364 disposed adjacent to the backing plate 324 of the inner pad 320 b,areaction portion 366 disposed adjacent to the backing plate 324 of theouter pad 320 a, and connecting portion 368 which extend over theperiphery of the disc rotor 312 so as to connect the presser andreaction portions 364, 366.

As shown in FIG. 19, a presser rod 370 slidably engages the presserportion 364, such that the front end face of the presser rod 370 facesthe backing plate 324 of the inner pad 320 b,for abutting contact withthis backing plate 324. On the back side of the presser rod 370, aultrasonic motor 372 is disposed coaxially with the presser rod 370. Theultrasonic motor 372 is fixed to the bracket 360 of the caliper 336. Thepresser rod 370 and the ultrasonic motor 372 are disposed such thattheir axes are parallel to the direction Y. Further, the presser rod 370and the ultrasonic motor 372 are operatively and coaxially connected toeach other through a ballscrew mechanism 374. A common axis L3 of thepresser rod 370, ultrasonic motor 372 and ballscrew mechanism 374 isparallel to the reference line L1 of the mounting bracket 330, and isoffset by a suitable distance from the reference line L1 in the rotatingdirection X of the disc rotor 312, as indicated in FIG. 20.

It will be understood from the above description of the ninth embodimentthat the inner pad 320 b is interposed between the disc rotor 312 andthe presser rod 370 such that the inner pad 320 b can be moved with thedisc rotor 312 due to the frictional contact of the slant surface 334with the friction surface 314, with the presser rod 370 held in abuttingcontact with the backing plate 324 of the inner pad 320 b. When theinner pad 320 b is moved with the disc rotor 312, the inner pad 320 bfunctions as a wedge, and the friction force generated between the innerpad 320 b and the disc rotor 312 is converted into an axial force whichacts on the disc rotor 312 and the presser rod 370 in oppositedirections so as to move the presser rod 370 away from the disc rotor312. Accordingly, the force by which the friction pads 320 are pressedagainst the opposite friction surfaces 314 of the disc rotor 312 isincreased, whereby the friction force between the inner pad 320 b andthe disc rotor 312 is increased. Thus, the dragging movement of theinner pad 320 b with the disc rotor 312 causes the self-servo effect.

The ultrasonic motor 372 is of a travelling-wave type. Since theprinciple of operation of this motor 372 is well known in the art, themotor 372 will be briefly described.

The motor 372 has a stator 382 and a rotor 384 which are coaxiallydisposed within a housing 380, as shown in FIG. 19. In operation, thestator 382 produces a surface wave upon application of a ultrasonicvibration thereto, and the rotor 384 is rotated with a friction forceacting between the stator 382 and the rotor 384.

The stator 382 consists of an elastic body 390 and a piezoelectric body392 both of which take the form of a ring. The elastic and piezoelectricbodies 390, 392 are superposed on each other and bonded together. On oneof the opposite surfaces of the piezoelectric body 392, two arcuatearrays of electrodes 392 a, 392 b are formed as shown in FIG. 24, suchthat the two arrays 392 a, 392 b have a phase difference of 90°. Eacharray 392 consists of a plurality of segment electrodes, for instance,nine segment electrodes, whose directions of polarization changealternately in a direction along the arc of the array. The two arcuatearrays 392 a, 392 b are spaced apart from each other by two areasadjacent to the opposite ends of each array 392. One of these two areasis provided with an electrode 392 c having a function described below.On the other surface of the piezoelectric body 392, there are formed twocommon electrodes 392 d, 392 e, which are connected to the respectiveelectrode arrays 392 a, 292 b. Namely, the common electrode 392 d isconnected to all of the segment electrodes of the array 392 a, while thecommon electrode 392 e is connected to all of the segment electrodes ofthe array 392 b.

The rotor 384 is forced by a pressing contactor mechanism 394 onto thestator 382, so that there is produced a suitable amount of frictionforce therebetween. The rotor 384 has a friction member bonded theretofor frictional contact with the stator 382, so that a travelling-wavevibration generated by the stator 382 is transmitted to the rotor 384,whereby the rotor 384 is rotated. A certain friction force existsbetween the stator 382 and the rotor 384 even when the piezoelectricbody 392 is in a de-energized or off state without a voltage applicationthereto by the pressing contactor mechanism 394. Based on this frictionforce, the motor 372 produces a holding torque. In the presentembodiment, the pressing contactor mechanism 394 is principallyconstituted by a coned disc spring 396. However, the coned disc spring396 may be replaced by a coil spring.

The ultrasonic motor 372 is provided with a rotary position sensor inthe form of an encoder 398 for detecting the rotary or angular positionof the rotor 394.

The ballscrew mechanism 374 indicated above includes an externallythreaded member (threaded shaft) 400, an internally threaded member(nut) 402, and a plurality of balls through which the externally andinternally threaded members 400, 402 engage each other. These threadedmembers 400, 402 are supported by a housing 380 such that the externallythreaded member 400 is not rotatable but is axially movable while theinternally threaded member 402 is rotatable but is not axially movable.Described in detail, the externally threaded member 400 has a splinedportion 404 splined to the housing 380 such that the member 400 is notrotatable relative to the housing 380, while the internally threadedmember 402 is attached to the housing 380 through a radial bearing 410and a thrust bearing 412 such that the member 402 is rotatable relativeto the housing 380. A stop 414 is provided to prevent an axial movementof the internally threaded member 402 relative to the housing 380. Tothis internally threaded member 402, there are attached the rotor 384and the pressing contactor mechanism 394 such that the rotor 384 and themechanism 394 are not rotatable relative to the housing 380. In thisarrangement, forward rotation of the internally threaded member 402 byforward rotation of the rotor 384 will cause the externally threadedmember 400 to move in the right direction as seen in FIG. 19, pushingthe presser rod 370 to be advanced for pressing the friction pads 320 tomove toward the disc rotor 312. Conversely, reverse rotation of theinternally threaded member 402 by reverse rotation of the rotor 384 willcause the externally threaded member 400 to move in the left directionas seen in FIG. 19, permitting the presser rod 370 to be retracted andthereby permitting the friction pads 320 to be retracted away from thedisc rotor 312.

The externally threaded member 400 is provided on its end face with aload sensor 420 concentrically attached thereto. The externally threadedmember 400 is adapted to abut on the back surface of the presser rod 370through the load sensor 420, so that the force by which the inner pad320 b is pressed by the motor 372 through the ballscrew mechanism 374can be detected based on the output signal of the load sensor 420.

Referring to the block diagram of FIG. 26, there is shown an electriccontrol system of the present electrically operated braking system. Thecontrol system includes a primary brake controller 420 arranged tocontrol the motor-driven disc brake 310, more specifically, control theultrasonic motor 372 for regulating the force by which the inner pad 320b is pressed by the motor 372. This force will be referred to simply as“pressing force of the inner pad 320 b”. The controller 430 isprincipally constituted by a computer incorporating a CPU, a ROM and aRAM.

The primary controller 430 is connected at its input interface to apressing command controller 432 which is also principally constituted bya computer. The pressing command controller 432 is connected to anoperation information sensor 434, a vehicle state sensor 436 and a wheelstate sensor 438.

The operation information sensor 434 is adapted to obtain informationrelating to the operation of the vehicle by the vehicle operator, suchas the steering angle of the steering wheel, operating state (operatingforce and/or amount) of the brake operating member, and operating state(operating force and/or amount) of the accelerator pedal. The presentbraking system includes a brake pedal (not shown) as the brake operatingmember to be depressed by the vehicle operator, and a device forproducing a brake operating force corresponding to the operating stateof the brake pedal. The operation information sensor 434 includes atleast a sensor for detecting this brake operating force as the operatingstate of the brake operating member. The vehicle state sensor 436 isadapted to obtain information relating the state of the vehicle, such asthe running speed, lateral and longitudinal acceleration values of thevehicle body, and a yaw rate and a slip angle of the vehicle body. Thewheel state sensor 438 is adapted to obtain information relating to thestate of each vehicle wheel, such as the rotating speed, accelerationand slip ratio of the wheel.

The pressing command controller 432 applies to the primary brakecontroller 430 various commands for controlling at least one of the discbrakes 310 for the four wheels, so as to effect various controls such as“braking force distribution control”, “anti-lock pressure control”,“traction control”, “vehicle stability control” and “abrupt brakingcontrol”.

In the “braking force distribution control”, the pressing force of thedisc brake 310 for each wheel is controlled so as to establish anoptimum distribution of the braking forces for the front wheels to thosefor the rear wheels, to establish a deceleration value of the vehiclewhich corresponds to the brake operating force, and to prevent lockingof the rear wheels prior to locking of the front wheels. The brakingoperating force is detected by the operation information sensor 434(e.g., brake pedal depression force sensor). The “anti-lock pressurecontrol” is initiated when a locking tendency of a wheel is detected. Inthe anti-lock pressure control, the pressing force of the disc brake 310for the wheel in question is controlled so as to prevent an increase inthe locking tendency of the wheel. The locking tendency of each wheel isdetected based on at least the output signal of the wheel state sensor438 (e.g., wheel speed sensors). The “traction control” is initiatedwhen a spinning tendency of a drive wheel is detected during starting oracceleration of the vehicle. In the traction control, the pressing forceof the disc brake 310 for the wheel in question is controlled so as toprevent an increase in the spinning tendency of the wheel. The spinningtendency of each drive wheel is also detected based on at least theoutput signal of the wheel state sensor 438 (e.g., wheel speed sensors).The “vehicle stability control” is initiated when an understeeringtendency or an oversteering tendency of the vehicle is detected. In thevehicle stability control, the pressing force of at least one of thedisc brakes 310 for the right and left wheels is controlled to regulatea difference between the braking forces applied to the right and leftwheels, so as to prevent an increase in the understeering oroversteering tendency. The understeering or oversteering tendency isdetected by the vehicle state sensor 436. The “abrupt braking control”is effected when an abrupt brake is applied to the vehicle. In theabrupt braking control, the pressing force of the disc brake 310 foreach wheel is controlled so as to compensate for a shortage of the wheelbraking forces corresponding to a shortage of the brake operating force.The abrupt brake application is detected based the output signal of theoperation information sensor 434, more precisely, based on the outputsignal of a sensor for detecting the operating amount of the brakeoperating member. Namely, the abrupt brake application is detected whena rate of increase in the operating amount becomes higher than apredetermined upper limit, which is not reached during normal brakeapplication.

The primary brake controller 430 is also connected at its inputinterface to a brake switch 440 and an ignition switch 442.

The brake switch 440 is a sensor for detecting an operation of the brakepedal as the brake operating member. The brake switch 440 is on when thebrake pedal is depressed, and off when the brake pedal is not operated.The ignition switch 442 is a sensor for detecting starting of an engineof the vehicle. The ignition switch 442 is on when the engine isoperating, and off when the engine is off

The primary brake controller 430 is further connected at its inputinterface to the load sensor 420 and encoder 398 which have beendescribed.

The present braking system further includes a parking brake controller450, which is adapted to activate the disc brakes 310, upon operation ofa parking brake, for holding the vehicle in a parked or stationarystate. Like the primary brake controller 430, the parking brakecontroller 450 is principally constituted by a computer. The parkingbrake controller 450 is connected at its input interface to a parkingbrake switch 454, which is a sensor for detecting an operation of theparking brake. The parking brake switch 454 is on when the parking brakeis operated, and off when the parking brake is in the non-operatedstate.

The primary brake controller 430 and the parking brake controller 450are connected at their output interfaces to a motor driver circuit 454,which is provided for the ultrasonic motor 372 of the disc brake 310 foreach wheel of the vehicle. To this motor driver circuit 454, there areconnected the ultrasonic motor 372, and a DC power source 456 commonlyused for the disc brakes 310 for the four wheels.

Referring to the block diagram of FIG. 29, there are shown functionalelements of the motor driver circuit 454. That is, the motor drivercircuit 454 includes a drive signal generator 458, a power supply 460and a frequency tracer 462.

The signal generator 458 is connected to the output interfaces of themain brake controller 430 and parking brake controller 450, to receive amotor control signal. Based on the received motor control signal, thesignal generator 458 applies to the power supply 460 a drive signalwhich has a variable frequency. The drive signal is a high-frequencytwo-phase alternating signal with a phase difference of 90° between thetwo arrays of electrodes 392 a, 392 b of the ultrasonic motor 372. Thepower supply 460 is connected to a DC power source 456. Based on thedrive signal received from the signal generator 458, the power supply460 supplies controlled power to the electrode arrays 392 a, 392 b ofthe motor 372.

For improving the driving efficiency of the ultrasonic motor 372, thepiezoelectric body 392 is preferably driven at a resonance frequencythereof or a frequency close to the resonance frequency. The resonancefrequency of the piezoelectric body 392 varies with its temperature anda load of the motor 372. The frequency tracer 462 is provided to changethe frequency of the drive signal generated by the signal generator 458,in response to or following a change in the resonance frequency of thepiezoelectric body 392. The frequency tracer 462 is arranged to monitorthe oscillating state of the stator 382, on the basis of the outputsignal of the electrode 392 c, while utilizing f fact that the electrode392 c generates a voltage corresponding to an oscillation amplitude ofthe stator 382 due to a piezoelectric effect. Based on the monitoredstate of the stator 382, the frequency tracer 462 applies to the signalgenerator 458 a signal for optimizing the frequency of the drive signalto be applied to the power supply 460.

The primary brake controller 430 executes a brake control routineillustrated in the flow chart of FIG. 28, according to a program storedin the ROM of the computer.

Described briefly, the brake control routine includes step S15 which isimplemented upon activation of the disc brake 310 (when the brake switch440 is turned ON), to control the ultrasonic motor 372 so that an actualpressing force Fs of the inner pad 320 b is made equal to a desired ortarget value F*.

When the actual pressing force Fs is smaller than the desired value F*,the motor 372 is energized in a first direction with a forward drivesignal applied thereto, and is rotated in a forward direction, so thatthe actual pressing force Fs is increased.

If an increase in the actual pressing force Fs is no longer detectedeven with the forward drive signal being applied to the motor 372, themotor 372 is de-energized with an OFF signal applied thereto, and themotor 372 generates the holding torque, so that the motor 372,internally and externally threaded members 402, 400 and presser rod 370are locked. In this condition, the actual pressing force Fs is increasedowing to the wedge effect of the inner pad 320 b. To check if the actualpressing force Fs is not increased while the forward drive signal isapplied to the motor 372, the primary brake controller 430 determineswhether the amount of increase of the actual pressing force Fs(N)detected in the present cycle (of execution of the brake control routineof FIG. 28) as compared with the actual pressing force Fs(N−1) detectedin the last cycle is equal to or smaller than a predetermined firstreference value, which is set to be “zero”, for instance. If the amountof increase is equal to or smaller than the first reference value, it isdetermined that the actual pressing force Fs is no longer increased evenwhile the forward drive signal is applied to the motor 372.

If an increase in the actual pressing force Fs is continuously detectedwith the motor 372 turned off, that is, if the amount of increase of thepresent value Fs(N) from the last value Fs(N−1) is larger than apredetermined second reference value (which is set to be “zero”, forexample), the motor 372 is held off (held locked). If an increase in theactual pressing force is no longer detected, the forward drive signal isapplied to the motor 372.

When the actual pressing force Fs is larger than the desired value F*,the motor 372 is energized in a second direction with a reverse drivesignal being applied therefore, and is rotated in the reverse direction,so that the actual pressing force Fs is decreased.

When the actual pressing force Fs is equal to the desired value F*, themotor 372 is held off or de-energized with the OFF signal appliedthereto.

The brake control routine of FIG. 28 also includes step S18 which isimplemented when the brake operating member is released, to control themotor 372 for returning the presser rod 370 to a predetermined initialor fully retracted position.

The brake control routine will be described in detail by reference tothe flow chart of FIG. 28. The brake control routine is executed for thedisc brakes 310 for the four wheels, in a predetermined order, evenwhile the ignition switch 442 is off. The following description is basedon an assumption that the routine is repeatedly executed with apredetermined cycle time T for the same wheel.

The brake control routine is initiated with step S11 to determinewhether the ignition switch 442 is ON. This determination is effected onthe basis of the output signal of the ignition switch 442. If a negativedecision (NO) is obtained in step S11, one cycle of execution of theroutine is terminated.

If an affirmative decision (YES) is obtained in step S11, the controlflow goes to step S12 in which a PRESSER ROD INITIAL POSITION FLAG(which will be described) is reset to “0”. Step S12 is followed by stepS13 to diagnose the primary brake controller 430, the ultrasonic motor372 (brake actuator) of the disc brake 310 for the wheel in question,and the motor drive circuit 454 for the wheel in question. Then, thecontrol flow goes to step S14 to determine whether the brake switch 440is OFF. This determination is effected based on the output signal of thebrake switch 440. If the brake switch 440 is ON, that is, if a negativedecision (NO) is obtained in step S14, the control flow goes to step S15in which the ultrasonic motor 372 is controlled to control the pressingforce Fs of the disc brake 310. Step S15 is followed by step S16 inwhich the PRESSER ROD INITIAL POSITION flag is reset to “0”. Then, thecontrol flow goes back to step S14. Thus, steps S14-S16 are repeatedlyimplemented while the brake switch 440 is ON, namely, while the brakeoperating member is held depressed.

In step S15, a pad pressing control routine as illustrated in the flowchart of FIG. 29 is executed. This routine is repeatedly executed assteps S14-S16 are repeatedly implemented while the brake switch 440 isheld ON.

The pad pressing control routine of FIG. 29 is initiated with step S101in which a parking brake control signal for releasing the parking brakeis applied to the parking brake controller 450. As a result, the parkingbrake by the disc brake 310 for the wheel in question is released, asdescribed later in detail. Step S101 is followed by step S102 in which apressing force signal f_(in) corresponding to the wheel in question isreceived from the pressing command controller 432, and the desired valueF* of the pressing force of the inner pad 320 b of the disc brake 310for the wheel in question is obtained based on the received pressingforce signal f_(in). Then, the control flow goes to step S103 todetermine whether the desired pressing force value F* is not smallerthan zero and is not larger than a predetermined upper limit f_(max).That is, step S103 is implemented to determine whether the obtaineddesired pressing force value F* is abnormal or not. If a negativedecision (NO) is obtained in step S103, one cycle of execution of theroutine of FIG. 29 is terminated.

If an affirmative decision (YES) is obtained in step S103, the controlflow goes to step S104 in which the actual pressing force Fs(N) isdetected based on a load signal f received from the load sensor 420. Theforce Fs(N) detected in the present cycle of execution of the routine ofFIG. 29 represents a force by which the inner pad 320 b is pressedagainst the disc rotor 312 by the presser rod 370. Step S104 is followedby step S105 to determine whether the presently detected actual pressingforce Fs(N) is smaller than the desired value F*. However, step S105 maybe modified to determine whether the detected actual pressing forceFs(N) is smaller than a sum of the desired value F* and a predeterminedsmall value Δ.

There are three possible cases, namely, a first case wherein thedetected actual pressing force Fs(N) is smaller than the desired valueF*, a second case wherein the detected actual pressing force Fs(N) islarger than the desired value F*, and a third case wherein the detectedactual pressing force Fs(N) is equal to the desired value F*. Thesethree cases will be described in this order.

(1) Where the detected actual pressing force Fs(N) is

smaller than the desired value F*

In this case, an affirmative decision (YES) is obtained in step S105,and the control flow goes to step S106 to determine whether the forwarddrive signal is being applied to the motor 372, that is, whether themotor 372 is commanded to operate in the forward direction.

The graph of FIG. 30 shows a relationship between a time t at which theactual pressing force Fs is detected by the load sensor 420, and a timet′ at which the motor 372 is turned on and off. The motor drive signalis generated based on the presently detected actual pressing forceFs(N). Accordingly, the present motor drive signal is generated at atime t′ (N) which is slightly later than a time t(N) at which the actualpressing force Fs(N) is detected in the present control cycle, and thenext motor drive signal is generated at a time t′ (N+1) which isslightly later than a time t(N+1) at which the actual pressing forceFs(N+1) is detected in the next control cycle. If a control period T isdefined as a period between the times t(N) and t(N+1) at which theactual pressing force Fs is detected in the present and next controlcycles, the last generated motor drive signal is effective in an initialportion of the present control period T, and the present motor drivesignal is generated at a moment some time later than the beginning ofthe present control period T. Therefore, step S106 is provided todetermine whether the last generated forward drive signal is beingapplied to the motor 372.

If a negative decision (NO) is obtained in step S106, the control flowgoes to step S107 to determine whether the motor 372 is off. Where theforward drive signal is not applied to the motor 372 (the negativedecision is obtained in step S106), either the reverse drive signal isapplied to the motor 372 or the motor 372 is in the de-energized or offstate. If the motor 372 is off and an affirmative decision (YES) isobtained in step S107, the control flow goes to step S108 to determinewhether the actual pressing force Fs(N) detected in the present controlcycle is larger than the actual pressing force Fs(N−1) detected in thelast control cycle, that is, whether the actual pressing force Fs is inthe process of increasing. Where the routine of FIG. 29 is executed forthe first time, step S108 determines whether the actual pressing forceFs(1) detected in the first control cycle is larger than a value Fs(0)which is zero. The value Fs(0) is stored in the ROM of the primary brakecontroller 430.

If a negative decision (NO) is obtained in step S108, the control flowgoes to step S109 in which the motor 372 is commanded to operate in theforward direction with the forward drive signal applied thereto. In thiscase, one cycle of execution of the routine is terminated.

If an affirmative decision (YES) is obtained in step S106 in the presentcontrol cycle following the control cycle in which the forward drivesignal was applied to the motor 372 in step S109, the control flow goesto step S110 to determine whether the actual pressing force Fs(N)detected in the present control cycle is equal to or smaller than theactual pressing force Fs(N−1) detected in the last control cycle, thatis, determine whether the actual pressing force Fs has increased as aresult of the forward operation of the motor 372. The actual pressingforce Fs will increase as a result of the forward operation of the motor372 until the actual pressing force Fs has reached an upper limitcorresponding to a maximum drive force of the motor 372. After theactual pressing force Fs has reached the upper limit, the motor 372 isnot operated in the forward direction but is operated in the reversedirection even with the forward drive signal being applied to the motor372, whereby the actual pressing force Fs is no longer increased. Thus,the determination in step S110 is effected to determine whether theactual pressing force Fs(N) has reached the upper limit. If a negativedecision (NO) is obtained in step S110, that is, if the presentlydetected actual pressing force Fs(N) is larger than the last detectedactual pressing force Fs(N1−), the control flow goes to step S109 inwhich the forward drive signal remains applied to the motor 372. In thiscase, one cycle of execution of the routine is terminated.

While the present actual pressing force Fs(N) is larger than the lastactual pressing force Fs(N−1), that is, before the actual pressing forceFs(N) has increased to the upper limit owing to the self-servo effect,the negative decision (NO) is obtained in step S110, and the forwarddrive signal is continuously applied to the motor 372 in step S109. Whenthe maximum drive force of the motor 372 has been reached, the actualpressing force Fs(N) no longer increases, and the presently detectedactual pressing force Fs(N) becomes equal to the last detected actualpressing force Fs(N−1). In this case, an affirmative decision (YES) isobtained in step S110, and the control flow goes to step S111 in whichthe motor 372 is de-energized or turned off, so that the motor 372produces the holding torque which resists a reaction force transmittedfrom the friction pads 320.

In the example of FIG. 30 wherein the actual pressing force Fs(N)detected at time t(N) in the present control cycle remains the same asthe actual pressing force Fs(N−1) detected at time t(N−1) in the lastcontrol cycle. In this example, the OFF signal is applied to the motor372 at time t′ (N). As a result, the holding torque is produced by themotor 372, and the actual pressing force Fs increases, in the presenceof the holding torque of the motor 372 resisting the reaction force ofthe friction pads 320.

In the control cycle executed after the motor 372 is turned off in stepS111, the negative decision (NO) is obtained in step S106 while theaffirmative decision (YES) is obtained in step S107, so that the controlflow goes to step S108. Since the actual pressing force Fs(N) increasesowing to the self-servo effect in the presence of the holding torque ofthe motor 372, the affirmative decision (YES) is obtained in step S108,and the control flow goes to step S112 in which the motor 372 remains inthe off state.

The self-servo effect of the inner pad 320 b decreases as the timepasses after the motor 372 is turned off to produce the holding torque.Eventually, the presently detected actual pressing force Fs(N) remainsthe same as the last detected value Fs(N−1). In this case, the negativedecision (NO) is obtained in step S108, and the control flow goes tostep S109 in which the forward drive signal is applied to the motor 372.In this respect, it is noted that the self-servo effect may not beprovided even with the motor 372 held in the off state, for some reasonor other, for instance, due to a gap between the front end face of thepresser rod 370 and the back surface of the inner pad 320 b, which gapmay be created as a result of an advancing movement of the inner pad 320b toward the disc rotor 312. In this case, the forward operation of themotor 372 with the forward drive signal being applied thereto in stepS109 will cause an increase in the actual pressing force Fs(N).

The affirmative decision (YES) may be obtained in step S105 while thereverse drive signal is applied to the motor 372. In this case, thenegative decision (NO) is obtained in step S106 and also in step S107,so that the control flow goes to step S113 in which the motor 372 isfirst turned off and then commanded to operate in the forward direction.

Referring to the graph of FIG. 31, there is shown another example of achange in the actual pressing force Fs up to the desired value F*, uponoperation of the brake operating member for activating the disc brake310.

In the example of FIG. 31, the brake operating member is operated attime t0, and the desired value F* of the actual pressing force Fs isdetermined also at the time t0. As a result, the motor 372 is turned on,and the actual pressing force Fs is increased. During an initial periodt0−t1, the elastic control mechanism 340 inhibits the dragging of theinner pad 320 b with the disc rotor 312, for thereby inhibiting theinner pad 320 b from achieving the self-servo effect.

As a result of the increase of the actual pressing force Fs during theforward rotation of the motor 372, the friction force acting on thefriction pad 320 b becomes larger than the predetermined threshold(determined by the elastic control mechanism 340) at the time t1, sothat the inner pad 320 b is dragged with the disc rotor 312, achievingthe self-servo effect for rapidly increasing the actual pressing forceFs of the inner pad 320 b during a period t1−t2.

The drive force of the motor 372 has reached the maximum value at thetime t2, and the pressing force Fs remains constant. As a result, themotor 372 is turned off at time t3, producing the holding torque. Duringthe following period t3−t4, the actual pressing force Fs increasesagain, in the presence of the holding torque. The actual pressing forceFs reaches the desired value F* at time t4. Subsequently, the motor 372is controlled so as to maintain the actual pressing force Fs at thedesired value F*, as described later.

(2) Where the detected actual pressing force Fs(N) is

larger than the desired value F*

In this case, a negative decision (NO) is obtained in step S105, and thecontrol flow goes to step S114 to determine whether the actual pressingforce Fs(N) is equal to the desired value F*. Step S114 may be modifiedto determine whether the actual pressing force Fs(N) is not smaller thanthe desired value F* minus a predetermined small value Δ, and is notlarger the desired value F* plus the predetermined small value Δ. In thepresent case wherein the actual pressing force Fs(N) is larger than thedesired value F*, a negative decision (NO) is obtained in step S114, andthe control flow goes to step S115 to determine whether the reversedrive signal is applied to the motor 372 or the motor 372 is off. If anaffirmative decision (YES) is obtained in step S115, the control flowgoes to step S116 in which the reverse drive signal is applied to themotor 372, so that the actual pressing force Fs is decreased. One cycleof execution of the routine is terminated with step S116.

If the forward drive signal is applied to the motor 372, a negativedecision (NO) is obtained in step S115, and the control flow goes tostep step S117 in which the motor 372 is first turned off, and then thereverse drive signal is applied to the motor 372.

(3) Where the detected actual pressing force Fs(N) is

equal to the desired value F*

In this case, the negative decision (NO) is obtained in step S105 whilean affirmative decision (YES) is obtained in step S114, so that stepS118 is implemented to turn off the motor 372. One cycle of execution ofthe routine is terminated with step S118.

The operation of the primary brake controller 430 when the brake switch440 is ON has been described above. If the brake switch 440 is OFF, anaffirmative decision (YES) is obtained in step S14 in the brake controlroutine of FIG. 28. In this case, the control flow goes to step S17 todetermine whether the PRESSER ROD INITIAL POSITION flag is set at “1”.If this flag is set at “0”, a negative decision (NO) is obtained in stepS17, and the control flow goes to step S18 in which the presser rodinitial position control routine is executed. Step S18 is followed bystep S19 to determine whether the ignition switch 442 is OFF. If theignition switch 442 is ON, that is, if a negative decision (NO) isobtained in step S19, the control flow goes back to step S14. Therefore,steps S14 and S17-S19 are repeatedly implemented while the brake switch440 is OFF while the ignition switch 442 is ON, and the PRESSER RODINITIAL POSITION flag s set at “0”.

The presser rod initial position control routine executed in FIG. 18 isillustrated in the flow chart of FIG. 32. This routine of FIG. 18 isrepeatedly executed when steps S14 and S17-19 are repeatedly implementedin the brake control routine of FIG. 28.

The presser rod initial position control routine will be first describedbriefly.

The brake control routine of FIG. 28 may be formulated such that thepresser rod 370 is returned to its predetermined initial or fullyretracted position when the brake operating member is turned to itsnon-operated position. In other words, the initial position in which thepresser rod 370 is placed while the disc brake 310 is not operated maybe fixed. However, the friction members 322 of the friction pads 320wear as it is used. If the presser rod 370 is always returned to thepredetermined fixed initial or fully retracted position, the gap betweenthe front end face of the presser rod 370 and the back surface of theinner pad 320 b increases with an increase in the amount of wear of thefriction pads 320. The increased gap means an unnecessarily largemovement of the presser rod 370 from its initial 10 position (fullyretracted or non-operated position) to the point of abutting contactwith the inner pad 320 b. In view of this fact, step S18 is implementedin the brake control routine of FIG. 28. Namely, the presser rod initialposition control routine of FIG. 32 is executed, for changing theinitial position of the presser rod 370 in accordance with a change inthe position of the back surface of the inner pad 320 b due to anincrease in the amount of wear of the friction members 322.

Described more specifically, immediately after the brake operatingmember is returned to the non-operated position, the presser rod 370 isadvanced from the initial position until the front end face comes intoabutting contact with the back surface of the inner pad 320 b. The axialposition of the presser rod 370 at which this abutting contact takesplace is obtained. The abutting contact of the presser rod 370 with theinner pad 320 b is detected when the load detected by the load sensor420 increases to a predetermined value. This value is the minimum loadvalue that can be detected by the load sensor 420, or slightly largerthan this minimum load value. The axial position at which the abuttingcontact takes place is obtained on the basis of the output signal of theencoder 398 which represents the rotary or angular position of the motor372. It is noted that the axial position of the presser rod 370 at whichthe abutting contact takes place reflects not only the amount of wear ofthe inner pad 320 b but also the amount of wear of the outer pad 320 a.Then, the presser rod 370 is retracted by a predetermined distance fromthe axial position at which the abutting contact took place. To thisend, the motor 372 is rotated in the reverse direction by apredetermined angle ΔΘ corresponding to the predetermined distanceindicated above. Thus, the initial or fully retracted position of thepresser rod 370 is updated in accordance with the amount of wear of thefriction members 322 of the friction pads 320.

Referring to the flow chart of FIG. 32, the presser rod initial positioncontrol routine will be described in detail.

The routine of FIG. 32 is initiated with step S201 in which the desiredvalue F* of the pressing force Fs of the inner pad 320 b is obtained onthe basis of a pressing force signal f_(m) received from the pressingcommand controller 432. The pressing force signal f_(m) represents theabove-indicated minimum load value that can be detected by the loadsensor 420. When the brake switch 440 is OFF, the pressing force signalf_(m) is fed from the pressing command controller 432 to the primarybrake controller 430.

Then, step S202 is implemented to obtain the actual pressing force Fs onthe basis of the load signal f received from the load sensor 420. StepS202 is followed by step S203 to determine whether the actual pressingforce Fs obtained in step S202 is smaller than the desired value F*obtained in step S201. Namely, step S203 is provided to determinewhether the presser rod 370 is still spaced apart from the inner pad 320b. Step S203 may be modified to determine whether the actual pressingforce Fs is smaller than the desired value F* plus a predetermined smallvalue Δ. If an affirmative decision (YES) is obtained in step S203, thatis, if the presser rod 370 has not come into abutting contact with theinner pad 320 b, the control flow goes to step S204 in which the forwarddrive signal is applied to the motor 372, so that the presser rod 370 isadvanced. Then, the control flow goes back to step S202.

When the actual pressing force Fs has reached or exceeded the desiredvalue F*, a negative decision (NO) is obtained in step S203, and thecontrol flow goes to step S204 to determined whether the actual pressingforce Fs is equal to the desired value F*. This step S204 is provided todetermine whether the presser rod 370 has been brought into abuttingcontact with the inner pad 320 b to initiate an operation to press theinner pad 320 b against the disc rotor 312. If the actual pressing forceFs has exceeded the desired value F*, a negative decision (NO) isobtained in step S205, and the control flow goes to step S206 todetermine whether the forward drive signal is being applied to the motor372. If an affirmative decision (YES) is obtained in step S206, thecontrol flow goes to step S207 in which the motor 372 is first turnedoff, and then the reverse drive signal is applied to the motor 372.Then, the control flow goes back to step S202. If a negative decision(NO) is obtained in step S206, the control flow goes to step S208 inwhich the reverse drive signal is applied to the motor 372. In either ofthe above cases of steps S207 and S208, the presser rod 370 is retractedin the direction away from the inner pad 320 b. Step S208 is alsofollowed by step S202.

If the actual pressing force Fs is equal to the desired value F*, itmeans that the presser rod 370 has come into abutting contact with theinner pad 320 b. In this case, an affirmative decision (YES) is obtainedin step S205, and the control flow goes to step S209 to turn off themotor 372. Step S209 is followed by step S210 in which the rotary orangular position Θ of the rotor 384 of the motor 372 is detected on thebasis of the angular position Θ detected by the encoder 398. Then, thecontrol flow goes to step S211 in which the motor 372 is rotated in thereverse direction by the predetermined angle ΔΘ with the reverse drivesignal applied thereto. The predetermined angle ΔΘ corresponds to asuitable amount of gap which is provided between the friction pads 320and the friction surfaces 314 of the disc rotor 312 and which isnecessary to avoid frictional contact or dragging of the friction pads320 with the disc rotor 312. As a result of the reverse rotation of themotor 372 in step S211, the presser rod 370 is retracted by thepredetermined distance from the point of abutting contact thereof withthe inner pad 320 b to the updated initial or fully retracted position.Thus, the initial position of the presser rod 370 is updated. Step S211is followed by step S212 in which the PRESSER ROD INITIAL POSITION flagis set to “1”. This flag indicates that the presser rod 370 is placed inthe updated initial position when it is set at “1”, and indicates thatthe initial position of the presser rod 370 has not been updated. Onecycle of execution of the presser rod initial position control routineof FIG. 32 is terminated with step S212.

Upon one cycle of execution of the presser rod initial position controlroutine of FIG. 32, the control flow goes to step S19 of the brakecontrol routine of FIG. 28 to determine whether the ignition switch 442is OFF. If this switch 442 is ON, the negative decision (NO) is obtainedin step S19, and the control flow goes back to step S14. If the brakeswitch 440 is OFF, the affirmative decision (YES) is obtained in stepS14, and the control flow goes to step S17. Since the PRESSER RODINITIAL POSITION flag has been set to “1” in step S212 of the presserrod initial position control routine of FIG. 32, an affirmative decision(YES) is obtained in step S17, and the control flow goes to step S19,while skipping step S18 (presser rod initial position control routine ofFIG. 32). If the ignition switch 442 is turned off as the steps S14 andS17-S19 are repeatedly implemented, an affirmative decision (YES) isobtained in step S19, and one cycle of execution of the routine of FIG.28 is terminated.

Referring to the flow chart of FIG. 33, there will be described aparking brake control routine executed by the parking brake controller450 according to a program stored in the ROM.

The parking brake control routine of FIG. 33 is executed also when theignition switch 442 is OFF. The routine is initiated with step S301 todetermine whether the parking brake switch 452 is ON. If an affirmativedecision (NO) is obtained in step S301, the control flow goes to stepS302 in which the motor 372 is operated in the forward direction by apredetermined angle Θ_(PKB) with the forward drive signal appliedthereto. As a result, the presser rod 370 is advanced from the initialposition, to produce the pressing force Fs necessary to apply a suitableparking brake force to the wheel in question. Step S302 is followed bystep S303 to turn off the motor 372. As a result, the motor 372 producesthe holding torque for maintaining the disc brake 310 in the parkingbrake state, so that the parked vehicle is held stationary. One cycle ofexecution of the routine of FIG. 33 is terminated with step S303.

If the parking brake switch 452 is OFF, a negative decision (NO) isobtained in step S301, and the control flow goes to step S304 todetermine whether the parking brake release signal is present. If anegative decision (NO) is obtained in step S304, one cycle of executionof the routine is terminated. If an affirmative decision (YES) isobtained in step S304, the control flow goes to step S305 in which themotor 372 is rotated in the reverse direction with the reverse drivesignal applied thereto, so that the presser rod 370 is returned to theinitial position. Step S305 is followed by step S303 to turn off themotor 372, and one cycle of execution of the routine is terminated.

It will be understood from the foregoing description of the presentninth embodiment of the invention that a motor control device forcontrolling the ultrasonic motor 372 is constituted by the primary brakecontroller 430, pressing command controller 432, operation informationsensor 434, vehicle state sensor 436, wheel state sensor 438, brakeswitch 440, ignition switch 442, load sensor 420, parking brakecontroller 450, parking brake switch 452, motor driver circuit 454 andencoder 398. It will also be understood that insufficient increasepreventing means for preventing shortage of the amount of increase ofthe actual pressing force Fs is constituted by the ultrasonic motor 372,load sensor 420 and a portion of the primary brake controller 430assigned to implement steps S106-S112 of FIG. 29, while de-energizingmeans for de-energizing the motor 372 to produce a holding torque areconstituted by the load sensor and the portion of the primary brakecontroller 430 assigned to implement steps S106-S112, and that the loadsensor 420 serves as a sensor for detecting a value relating to thepressing force Fs, and a pressing force sensor for detecting thepressing force Fs.

Next, a tenth embodiment of this invention will be described. This tenthembodiment, which is similar in many aspects to the ninth embodiment, isdifferent from the ninth embodiment only in the pad pressing controlroutine. Further, the pad pressing control routine in the tenthembodiment is similar in many aspects from that in the ninth embodiment.Therefore, there will be described only the steps of the pad pressingcontrol routine of the tenth embodiment which are different from thoseof the ninth embodiment, with the same step numbers being used toidentify the same steps.

In the ninth embodiment, the affirmative decision (YES) is obtained insteps S105 and S107 while the negative decision (NO) is obtained in stepS106 if the actual pressing force Fs(N) is smaller than the desiredvalue F* while the motor 372 is in the off state. In this case, thecontrol flow goes to step S108 to determine whether the presentlydetected actual pressing force Fs(N) has increased with respect to thelast detected value Fs(N−1). The control flow goes to step S109 or S112depending upon the affirmative or negative decision obtained in stepS108. If the presently detected actual pressing force Fs(N) has notincreased, that is, if the negative decision (NO) is obtained in stepS108, it means that it is not appropriate to hold the motor 372 in theoff state, so that the forward drive signal is applied to the motor 372in step S109. If the presently detected actual pressing force Fs(N) hasincreased, that is, if the affirmative decision (YES) is obtained instep S108, it means that it is appropriate to hold the motor 372 in theoff state, so that the motor 372 is held off in step S112.

The pad pressing control routine according to the tenth embodiment isillustrated in the flow chart of FIG. 34. This routine of FIG. 34 doesnot include steps S108 and S112. Accordingly, if the presently detectedactual pressing force Fs(N) is smaller than the desired value F* whilethe motor 372 is off, the affirmative decision (YES) is obtained insteps S105 and S107 while the negative decision (NO) is obtained in stepS106, the control flow goes to step S109 to apply the forward drivesignal to the motor 372, regardless of whether the presently detectedactual pressing force Fs(N) has increased with respect to the lastdetected value Fs(N−1). Thus, when the affirmative decision (YES) isobtained in step S107, step S109 is necessarily implemented to apply theforward drive signal to the motor 372. In the event where the presentlydetected actual pressing force Fs(N) has not increased with respect tothe last detected value Fs(N−1) due to inappropriate operation of themotor 372 in the forward direction, the motor 372 is subsequently turnedoff in the next execution of the present routine. That is, theaffirmative decision (YES) is obtained not only in step S106 but also instep S110, so that step S111 is implemented to turn off the motor 372.

As described above, the present ninth embodiment of the invention isadvantageous in that the pad pressing control routine of FIG. 34 issimpler than that of FIG. 29 according to the ninth embodiment.

It will be understood from the above description of the tenth embodimentthat the insufficient increase preventing means for preventing shortageof the amount of increase of the pressing force is constituted by theultrasonic motor 372, the load sensor 420 and a portion of the primarybrake controller 430 assigned to implement steps S106, S107 andS109-S111 of FIG. 34, while the de-energizing means for de-energizingthe motor 372 to produce a holding torque are constituted by the loadsensor 420 and the portion of the primary brake controller 430 assignedto implement steps S106, S107 and S109-S111.

Then, an eleventh embodiment of this invention will be described. Thiseleventh embodiment, which is also similar in many aspects to the ninthembodiment, is different from the ninth embodiment in the pad pressingcontrol routine. Further, the pad pressing control routine in theeleventh embodiment is similar in many aspects from that in the ninthembodiment. Therefore, there will be described only the steps of the padpressing control routine of the eleventh embodiment which are differentfrom those of the ninth embodiment, with the same step numbers beingused to identify the same steps.

The pad pressing control routine according to the eleventh embodiment isillustrated in the flow chart of FIG. 35. Described briefly, the presentroutine is formulated such that the motor 372 is first turned on andthen turned off within the same cycle of execution, irrespective ofwhether the drive force of the motor 372 has reached the maximum value,except in the case where the presently detected actual pressing forceFs(N) is equal to the desired value F*. According to this arrangement,the presser rod 370 is stopped after it is once advanced or retracted,except in the above-indicated case. Thus, the present routine does notrequire the motor 372 to be turned off each time the motor 372 isoperated in the reverse direction, whereby the routine is made simpler.

The pad pressing control routine of FIG. 35 is further formulate suchthat the ON time of the motor 372 is changed depending upon whether theactual pressing force Fs is increasing or not, so that the presser rod370 is positively advanced when the pressing force Fs is increasing, butis negatively advanced when the pressing force Fs is not increasing.Described in detail referring to the graphs of FIG. 36, the forwarddrive signal is applied to the motor 372 during an initial portion T1 ofthe control period T when the actual pressing force Fs is increasing, asindicated at (a) in the figure. The motor 372 is held off during theremaining portion (T−T1) of the control period T. When the actualpressing force Fs is not increasing, the forward drive signal is appliedto the motor 372 during an initial portion T2 of the control period T,which portion T2 is shorter than the portion T1, as indicated at (b) inFIG. 36. The motor 372 is held off during the remaining portion (T−T2)of the control period T.

As described above, the present eleventh embodiment is adapted such thatwhere an increase in the actual pressing force Fs is no longer detectedeven with the motor 372 being operated, the motor 372 is turned off fora given length of time within the control period T so that the motor 372produces the holding torque for locking the presser rod 370, whereby theactual pressing force Fs is increased. Further, when an increase in theactual pressing force Fs is no longer detected while the motor 372 isheld off, the forward drive signal is applied to the motor 372 for agiven length of time within the control period T, so that the presserrod 370 is advanced with the forward rotation of the motor 372, wherebythe friction pads 320 are forced against the disc rotor 312 by thepresser rod 370, and the actual pressing force Fs is increased.

The pad pressing control routine according to the eleventh embodiment ofthe invention will be described in detail referring to the flow chart ofFIG. 35.

The routine of FIG. 35 is initiated with step S101, which is followed bysteps S102-S105. These steps S101-S105 are identical with those in theninth embodiment of FIG. 29. If the presently detected actual pressingforce Fs(N) is smaller than the desired value F*, that is, if theaffirmative decision (YES) is obtained in step S105, the control flowgoes to step S131 to determine whether the presently detected actualpressing force Fs(N) is larger than the last detected value Fs(N−1),namely, whether the actual pressing force Fs is increasing. If anaffirmative decision (YES) is obtained in step S131, the control flowgoes to step S132 in which the forward drive signal is applied to themotor 372 for the time length T1. Step S132 is followed by step S133 inwhich the motor 372 is turned off. One cycle of execution of the routineis terminated with step S133.

If a negative decision (NO) is obtained in step S131, the control flowgoes to step S134 in which the forward drive signal is applied to themotor 372 for the time length T2. Step S134 is followed by step S133 toturn off the motor 372, and one cycle of execution of the routine isterminated.

If the detected actual pressing force Fs(N) is larger than the desiredvalue F*, a negative decision (NO) is obtained in step S105 and also instep S114, and the control flow goes to step S135 in which the reversedrive signal is applied to the motor 372 for the time length T3, whichmay be equal to the time length T1. Step S135 is followed by step S133,and one cycle of execution of the routine is terminated.

If the detected actual pressing force Fs(N) is equal to the desiredvalue F*, the negative decision (NO) is obtained in step S105, and anaffirmative decision (YES) is obtained in step S114. In this case, themotor 372 is turned off in step S133, and one cycle of execution of theroutine is terminated.

It will be understood from the above description of the eleventhembodiment that the insufficient increase preventing means forpreventing shortage of the amount of increase of the pressing force Fsis constituted by the ultrasonic motor 372, the load sensor 420 and aportion of the primary brake controller 430 assigned to implement stepsS131-S134 of FIG. 35, while the de-energizing means for de-energizingthe motor 372 to produce a holding torque are constituted by the loadsensor 420 and the portion of the primary brake controller 430 assignedto implement steps S131-S134.

There will next be described a twelfth embodiment of this invention,which is also similar in various aspects to the ninth embodiment. Thistwelfth embodiment is different from the ninth embodiment only in thepad pressing control routine. Further, the pad pressing control routinein the twelfth embodiment is similar in some aspects from that in theninth embodiment. Therefore, there will be described only the steps ofthe pad pressing control routine of the twelfth embodiment which aredifferent from those of the ninth embodiment, with the same step numbersbeing used to identify the same steps.

In the ninth embodiment, the motor 372 is turned off when an increase ofthe pressing force Fs is no longer detected. In the present twelfthembodiment, however, the motor 372 is turned off upon detection of theself-servo effect provided by the inner pad 320 b, without determiningwhether the presently detected actual pressing for Fs(N) is larger thanthe last detected value Fs(N−1). To detect the self-servo effect, theROM of the primary brake controller 430 stores a program for executing aself-servo effect monitoring routine for determining whether theself-servo effect is presently provided. This determination is effectedby determining whether the rate of increase of the actual pressing forceFs is higher than a predetermined threshold. This threshold isdetermined such that the rate of increase does not exceed the thresholdwhile the self-servo effect is not provided.

The pad pressing control routine executed in the present twelfthembodiment is illustrated in the flow chart of FIG. 37. This routine,which is repeatedly executed, is initiated with step S101 followed bysteps S102-S105, as in the routine of FIG. 29 of the ninth embodiment.

If the affirmative decision (YES) is obtained in step S105 with theactual pressing force Fs(N) being larger than the desired value F*, thecontrol flow goes to step S151 to execute the self-servo effectmonitoring routine indicated above.

The self-servo effect monitoring routine is illustrated in the flowchart of FIG. 38.

This self-servo effect monitoring routine is initiated with step S401 todetermine whether the actual pressing force Fs(N) is larger than areference value f_(c) which corresponds to the pre-load of the elasticcontrol mechanism 350. This reference value f_(c) is a pressing forcevalue Fs at which the self-servo effect begins to be provided. If theactual pressing force Fs(N) is not larger than the reference valuef_(c), that is, if a negative decision (NO) is obtained in step S401,the control flow goes to step S402 in which a SELF-SERVO flag is resetto “0”. One cycle of execution of the routine of FIG. 38 is terminatedwith step S402.

If the actual pressing force Fs(N) is larger than the reference valuef_(c), that is, if an affirmative decision (YES) is obtained in stepS401, the control flow goes to step S403 to determine whether theforward drive signal is being applied to the motor 372. If a negativedecision (NO) is obtained in step S403, the control flow goes to thestep S402 described above. If an affirmative decision (YES) is obtainedin step S403, the control flow goes to step S404 to calculate an amountof change ΔFs of the presently detected actual pressing force Fs(N) withrespect to the last detected value Fs(N−1). Step S404 is followed bystep S405 to determine whether the calculated amount of change ΔFs islarger than a predetermined reference value Δf_(s). This reference valueΔf_(s) is an amount of increase of the actual pressing force Fs duringthe cycle time of the routine of FIG. 38, which takes place due to anadvancing movement of the presser rod 370 by the forward rotation of themotor 372 while the self-servo effect is not provided. If an affirmativedecision (YES) is obtained in step S405, the control flow goes to stepS406 in which the SELF-SERVO flag is set to “1”. One cycle of executionof the routine is terminated with step S406. If a negative decision (NO)is obtained in step S405, the control flow goes to the step S402 toreset the flag to “0” as described above.

The self-servo effect monitoring routine of FIG. 38 may be modified toeffect only one of the two determinations of steps S401 and S405, fordetecting the self-servo effect when the affirmative decision isobtained in the determination effected. However, the present routine ofFIG. 38 adapted to effect both of the determinations of steps S401 andS405 permits higher accuracy of determination of the self-servo effect.That is, the state of the disc brake 310 in which the self-servo effectis provided is detected only when the actual pressing force Fs(N) islarger than the reference value f_(c) and when the amount of increaseΔFs of the force Fs is larger than the reference value Δf_(s).

Upon completion of the self-servo effect monitoring routine in step S151of the pad pressing control routine of FIG. 37, the control flow goes tostep S152 to determine whether the SELF-SERVO flag is set at “1”. If anegative decision (NO) is obtained in step S152, it means that theself-servo effect is not provided. In this case, the control flow goesto step S153 to determine whether the forward drive signal is beingapplied to the motor 372 or the motor 372 is off. If an affirmativedecision (YES) is obtained in step S153 with the motor 372 being off,the control flow goes to step S154 in which the forward drive signal isapplied to the motor 372. If the reverse drive signal is being appliedto the motor 372, a negative decision (NO) is obtained in step S153, andthe control flow goes to step S156 in which the motor 372 is firstturned off, and the forward drive signal is then applied to the motor372. One cycle of execution of the routine of FIG. 37 is terminated withstep S154 or S155.

If the self-servo effect has been detected in the self-servo effectmonitoring routine of FIG. 38, that is, if the SELF-SERVO flag has beenset to “1” in step S406, an affirmative decision (YES) is obtained instep S152. In this case, the control flow goes to step S156 to turn offthe motor 372, and one cycle of execution of the routine is terminated.Unlike the routine of FIG. 29 of the ninth embodiment, the routine ofFIG. 37 of the present twelfth embodiment does not include stepscorresponding to the S107, S107 and S113. In this respect, it is notedthat the self-servo effect monitoring routine of FIG. 38 is formulatedto set the SELF-SERVO flag to “1” only when the forward drive signal ispresent. Therefore, it is known that the forward drive signal is presentwhen the affirmative decision (YES) is obtained in step S152 of the padpressing control routine of FIG. 37. The reverse drive signal is notpresent when the affirmative decision is obtained in step S152.Accordingly, the steps S107, S107 and S113 of the routine of FIG. 29 arenot necessary in the routine of FIG. 37.

While the operation according to the routine of FIG. 37 where the actualpressing force Fs(N) is larger than the desired value F* has beendescribed above, the operation where the force Fs(N) is equal to orsmaller than the desired value F* is the same as in the ninthembodiment.

Referring to the graph of FIG. 39, there is indicated a change of theactual pressing force Fs when the motor 372 is operated in the forwarddirection. As in the example of FIG. 31, the brake operating member isoperated at point of time t10, and the desired value F* of the pressingforce Fs is determined. Subsequently, the actual pressing force Fsincreases up to the desired value F*.

During the period between the points of time t10 and t11, the draggingmovement of the inner pad 320 b with the disc rotor 312 is inhibited bythe elastic control mechanism 340, to inhibit the inner pad 320 b fromproviding the self-servo effect. The actual pressing force Fs isincreased as the motor 372 is operated in the forward direction. At thepoint of time t11, the friction force of the inner pad 320 b becomeslarger than the pre-load or initial biasing force of the elastic controlmechanism 340, and the inner pad 320 b is dragged with the disc rotor312, providing the self-servo effect.

When the self-servo effect is detected, the motor 372 is turned off,irrespective of whether the actual drive force of the motor 372 hasreached its maximum value. As a result, the actual pressing force Fs iscontinuously increased owing to the holding torque of the motor 372 andthe edge effect of the inner pad 320 b, until the force Fs has reachedthe desired value F* at point of time t12. Then, the motor 372 iscontrolled so as to maintain the actual pressing force Fs at the desiredvalue F*.

It will be understood from the above description of the twelfthembodiment that the insufficient increase preventing means forpreventing shortage of the amount of increase of the pressing force Fsis constituted by the ultrasonic motor 372, the load sensor 420 and aportion of the primary brake controller 430 assigned to implement stepsS151-S156 of FIG. 37, while the de-energizing means for de-energizingthe motor 372 to produce a holding torque are constituted by the loadsensor 420 and the portion of the primary brake controller 430 assignedto implement steps S151-S156. It will also be understood that self-servoeffect monitoring means for monitoring the self-servo effect of theinner pad 320 b is constituted by a portion of the primary brakecontroller 430 assigned to execute the self-servo effect monitoringroutine of FIG. 38 (implement step S151 of the routine of FIG. 37).

In the ninth through twelfth embodiments described above, the motor 372is turned off to produce the holding force for increasing the actualpressing force F. When it becomes necessary to reduce the desired valueF* of the pressing force Fs while the motor 372 is off, the drive signalis applied to the motor 372. In this case, the motor 372 may not beoperated with a sufficiently high response. Where it is important toprevent such a delayed operation of the motor 372, it is desirable tokeep applying the forward drive signal to the motor 372 rather than turnoff the motor for producing the holding torque, even when it isnecessary to increase the actual pressing force Fs.

A thirteenth embodiment of this invention will be described next. Thisthirteenth embodiment, which is similar in many aspects to the ninthembodiment, is different from the ninth embodiment only in the padpressing control routine. Further, the pad pressing control routine inthe thirteenth embodiment is similar in some aspects from that in theninth embodiment. Therefore, there will be described only the steps ofthe pad pressing control routine of the thirteenth embodiment which aredifferent from those of the ninth embodiment, with the same step numbersbeing used to identify the same steps.

The actual pressing force Fs is increased with the self-servo effect, byeither operating the motor 372 in the forward direction before themaximum drive force of the motor 372 is reached, or turning off themotor 372 after the maximum drive force of the motor is reached.According to this arrangement, the rate of increase of the actualpressing force Fs is comparatively high. Therefore, where an amount ofshortage ΔF of the actual pressing force Fs with respect to the desiredvalue F* is relatively small, the actual pressing force Fs may berapidly increased to a value considerably exceeding the desired value F*where the force Fs is increased by turning off the motor 372.

The actual pressing force Fs is decreased by operating the motor 372 inthe reverse direction while the self-servo effect is provided. The rateof decrease of the actual pressing force Fs is comparatively high likethe rate of increase indicated above. Therefore where an amount ofexcess ΔF′ of the actual pressing force Fs with respect to the desiredvalue F* is relatively small, the actual pressing force Fs may berapidly decreased to a value considerably smaller than the desired valueF* where the force Fs is decreased by operating the motor 372 in thereverse direction.

While the self-servo effect is not provided, the actual pressing forceFs is maintained at the same value by turning off the motor 372 with theinner pad 320 b held in contact with the disc rotor 312. While theself-servo effect is provided, on the other hand, the actual pressingforce Fs is increased with the self-servo effect, even with the motor372 being off. According, some measure should be taken to maintain theactual pressing force Fs while the self-servo effect is provided.

In the light of the above analysis, the present thirteenth embodimentemploys force increasing control means, force decreasing control meansand force holding control means for controlling the rate of change ofthe actual pressing force Fs, for increasing, decreasing and maintainingthe actual pressing force Fs, respectively, while the self-servo effectis provided.

As described above, the motor driver circuit 454 is adapted such thatthe drive frequency of the drive signal supplied to the motor 372 iscontrolled so as to follow a change in the resonance frequency of thestator 382. In the present thirteenth embodiment, the drive frequency ofthe motor 372 is controlled in the manner described below.

The frequency tracer 462 and the drive signal generator 458 cooperate torepeatedly detect an optimum drive frequency of the motor 372 duringoperation of the motor 372, by first raising the drive frequency to alevel which is higher by a suitable amount than an expected value of theoptimum drive frequency, and then lowering the drive frequency from thatlevel to the optimum level.

In a first step of detection, the drive frequency of the motor 372 israised to a level which is too high to operate the motor 372. Then, thedrive frequency is lowered at a predetermined rate from that level downto the expected optimum value, as indicated in the graph of FIG. 40. Inthe first step, a predetermined initial value is used as this expectedoptimum value. If the drive torque of the motor 372 exceeds a startingtorque while the drive frequency is lowered, the motor 372 is started.During the lowering of the drive frequency, the stator 382 is monitoredto check if its oscillating state coincides with a reference state(e.g., resonance state), depending upon the output signal of theelectrode 392 c. If the reference state of the stator 382 is detected,the lowering of the drive frequency is terminated, and the drivefrequency at that time is determined as the next expected optimum value.Namely, the expected optimum value of the drive frequency is updated.

In a second step of detection, the drive frequency is first raised to alevel which is higher by a given amount than the updated expectedoptimum value, and is then lowered by the predetermined rate to thatexpected optimum value. As in the first step, the stator 382 ismonitored during the lowering of the drive frequency, on the basis ofthe output signal of the electrode 392 c, to determine if theoscillation state coincides with the reference state. If the referencestate is detected, the lowering of the drive frequency is terminated,and the drive frequency at that time is determined as the next expectedoptimum value.

The second step described above is repeated until the motor 372 isturned off, so that the drive frequency of the motor 372 is controlledto the optimum value which changes with variations in the temperature ofthe stator 382, load acting on the motor 372, etc. Therefore, the motor372 can always be operated with high efficiency.

Generally, the motor 372 has a characteristic that its drive torqueincreases with a decrease in the drive frequency, when the drivefrequency is higher than the resonance frequency of the stator 382.Accordingly, immediately after the beginning of the first detectionstep, the drive torque of the motor 372 is too small to rotate the motor372, even with the drive signal being applied to the motor 372. If therate of lowering of the drive frequency is reduced, the rate of increaseof the drive torque is accordingly reduced, and the period for which thedrive torque is small is elongated. In the graph of FIG. 40, arelatively high rate of initial lowering of the drive frequency in thefirst detection step is indicated at A, while a relatively low rate ofinitial lowering of the drive frequency is indicated at B. Where thedrive frequency is lowered at the relatively high rate, the motor 372 isstarted at a point of time t1. Where the drive frequency is initiallylowered at the relatively low rate, the motor 372 is stared at a pointof time t2.

Therefore, when it is required to reduce the actual pressing force Fs ata relatively high rate, the motor 372 is operated in the reversedirection, and the drive frequency of the motor 372 is initially loweredat a normal rate V0. When it is required to reduce the actual pressingforce Fs at a relatively low rate, the motor 372 is operated in thereverse direction, and the drive frequency is initially lowered at afirst rate V1 which is lower than the normal rate V0.

When it is required to increase the actual pressing force Fs at arelatively high rate with the self-servo effect, the motor 372 is turnedoff. If the motor 372 is operated in the reverse direction with thedrive frequency being initially lowered at the normal rate V0 in orderto increase the actual pressing force Fs at a relatively low rate, theactual pressing force is in fact reduced. Hence, when it is required toincrease the actual pressing force Fs at a relatively low rate, themotor 372 is operated in the reverse direction, and the drive frequencyis initially lowered at a second rate V2 which is lower than the normalrate V0. The second rate V2 may or may not be equal to the first rateV1.

When it is required to maintain the actual pressing force Fs, this forceFs is in fact increased if the motor 372 is held off while theself-servo effect is provided. The actual pressing force Fs is decreasedif the motor 372 is operated in the reverse direction with the drivefrequency being initially lowered at the normal rate V0. The actualpressing force Fs is slightly decreased if the motor 372 is operated inthe reverse direction with the drive frequency being initially loweredat the second rate V2. Therefore, when it is required to maintain theactual pressing force Fs, the motor 372 is operated in the reversedirection with the drive frequency being initially lowered at a thirdrate V3 which s lower than the second rate V2.

In the present thirteenth embodiment, a pad pressing control routine isformulated in the light of the above-indicated finding. This routine isillustrated in the flow chart of FIG. 41, and includes step S160 inwhich a force decreasing control routine is executed to control thedrive frequency of the motor 372 when the actual pressing force Fs isdecreased. The force decreasing control routine is illustrated in theflow chart of FIG. 42. The pad pressing control routine incorporatessteps for controlling the drive frequency of the motor 372 when theactual pressing force Fs is increased and maintained.

The pad pressing control routine of FIG. 41 is repeatedly executed, andincludes the steps S101-S105 described above with respect to the ninthembodiment of FIG. 29 (twelfth embodiment of FIG. 37).

If the actual pressing force Fs(N) is smaller than the desired value F*,that is, if the affirmative decision (YES) is obtained in step S105, thecontrol flow goes to step S151 in which the self-servo effect monitoringroutine of FIG. 38 is executed. Step S151 is followed by step S152 todetermine whether the SELF-SERVO flag is set at “1”. If the flag is setat “0”, that is, if the negative decision (NO) is obtained in step S142,the control flow goes to steps S153-S155 described above with respect tothe routine of FIG. 37. If the affirmative decision (YES) is obtained instep S153 with the motor 372 being off, the drive frequency of the motor372 is initially lowered at the normal rate V0 in step S154. Namely, theinitial rate of lowering of the drive frequency is set to V0. If theaffirmative decision (YES) is obtained in step S153 with the forwarddrive signal being applied to the motor 372, the drive frequency isinitially lowered at the normal rate V0 in step S155, as in step S154.One cycle of execution of the routine of FIG. 41 is terminated with stepS154 or S155.

If the SELF-SERVO flag is set at “1” and the affirmative decision (YES)is obtained in step S152, the control flow goes to step S157 tocalculate the amount of shortage ΔF of the actual pressing force Fs(N)with respect to the desired value F*. Step S157 is followed by step S158to determine whether the calculated amount of shortage ΔF is larger thana predetermined reference value f_(a). If an affirmative decision (YES)is obtained in step S158, the control flow goes to step S156 to turn offthe motor 372 so that the actual pressing force Fs is rapidly increased.If a negative decision (NO) is obtained in step S158, the control flowgoes to step S159 in which the motor 372 is first turned off, and themotor 372 is commanded to operate in the reverse direction with thereverse drive signal applied thereto. In this case, the drive frequencyof the motor 372 is initially lowered at the second rate V2. That is,the initial rate of lowering of the drive frequency is set to V2 in stepS159. As a result, the actual pressing force Fs is slowly increased. Onecycle of execution of the routine is terminated with step S156 or S159.

If the actual pressing force Fs(N) is smaller than the desired value F*,the negative decision (NO) is obtained in step S105, and a negativedecision (NO) is obtained in step S114, so that step S160 is implementedto execute the force decreasing control routine illustrated in the flowchart of FIG. 42.

The force decreasing control routine of FIG. 42 is initiated with stepS501 in which the self-servo effect monitoring routine is executed asillustrated in FIG. 38. Step S501 is followed by step S502 to determinewhether the SELF-SERVO flag is set at “1”. If the negative decision (NO)is obtained in step S502, the control flow goes to step S503 todetermine whether the reverse drive signal is being applied to the motor372 or the motor 372 is off. If the affirmative decision (YES) isobtained in step S403, the control flow goes to step S504 in which thereverse drive signal is applied to the motor 372, with the drivefrequency being initially lowered at the normal rate Vo. That is, theinitial rate of lowering of the drive frequency is set to V0 if thereverse drive signal is applied to the motor 372 or if the motor 372 isin the off state. If the forward drive signal is being applied to themotor 372 and the negative decision (YES) is obtained in step S503, thecontrol flow goes to step S505 in which the motor 372 is first turnedoff, and the reverse drive signal is then applied to the motor 372. Inthis case, too, the initial rate of lowering of the drive frequency isset to V0. One cycle of execution of the routine of FIG. 42 isterminated with step S504 or S505.

When the self-servo effect is provided with the SELF-SERVO flag set at“1”, an affirmative decision (YES) is obtained in step S502, and thecontrol flow goes to step S506 to calculate the amount of excess ΔF′ ofthe actual pressing force Fs(N) with respect to the desired value F*.Step S506 is followed by step S507 to determine whether the calculatedamount of excess ΔF′ is larger than a predetermined reference valuef_(g). If an affirmative decision (YES) is obtained in step S507, thecontrol flow goes to steps S503-S505 described above, and the drivefrequency of the motor 372 is initially lowered at the normal rate V0,so that the actual pressing force Fs is rapidly decreased. If the amountof excess ΔF′ is not larger than the reference value f_(g), a negativedecision (NO) is obtained in step S507, and the control flow goes tostep S508 to determine whether the motor 372 is off. If an affirmativedecision (YES) is obtained in step S508, the control flow goes to stepS509 in which the reverse drive signal is applied to the motor 372, withthe drive signal being initially lowered at the first rate V1, so thatthe actual pressing force Fs is slowly decreased. If the negativedecision (NO) is obtained in step S508, the control flow goes to stepS510 in which the motor 372 is first turned off, and the reverse drivesignal is then applied to the motor 372 with the drive frequency beinginitialled lower at the first rate V1, so that the actual pressing forceFs is slowly decreased. One cycle of execution of the routine of FIG. 42is terminated with step S509 or S510.

Where the actual pressing force Fs(N) is equal to the desired value F*,the negative decision (NO) is obtained in step S105 of the pad pressingcontrol routine of FIG. 41, and the affirmative decision (YES) isobtained in step S114, so that step S161 is implemented to execute theself-servo effect monitoring routine of FIG. 38. Step S161 is followedby step S162 to determine whether the SELF-SERVO flag is set at “1”. Ifthe negative decision (NO) is obtained in step S162, the control flowgoes to step S163 to turn off the motor 372, and one cycle of executionof the routine of FIG. 41 is terminated.

If the affirmative decision (YES) is obtained in step S162, the controlflow goes to step S164 to determine whether the motor 372 is off. If theaffirmative decision (YES) is obtained in step S164, the control flowgoes to step S164 in which the reverse drive signal is applied to themotor 372, with the drive frequency being initially lowered at the thirdrate V3. If the negative decision (NO) is obtained in step S164, thecontrol flow goes to step S166 in which the motor 372 is first turnedoff, and the reverse drive signal is then applied to the motor 372 withthe drive frequency being initially lowered at the third rate V3. Inthese cases of steps S165 and S166, the actual pressing force Fs ismaintained. One cycle of execution of the routine of FIG. 41 isterminated with step S165 or S166.

It will be understood from the above description of the thirteenthembodiment that the insufficient increase preventing means forpreventing shortage of the amount of increase of the pressing force Fsis constituted by the ultrasonic motor 372, the load sensor 420 and aportion of the primary brake controller 430 assigned to implement stepsS151-S159 of FIG. 41. It will also be understood that the self-servoeffect monitoring means is constituted by a portion of the primary brakecontroller 430 assigned to implement step S151, S161 and S501 (executethe self-servo effect monitoring routine of FIG. 38).

The illustrated embodiments are adapted to determine whether it isnecessary to control the ultrasonic motor 372 for increasing the actualpressing force Fs. This determination is effected by utilizing: thetechnique wherein a determination as to whether the amount of increaseof the actual pressing force Fs becomes smaller than a first thresholdvalue in the first state of the motor 372 is effected to determinewhether the drive force of the motor has reached the maximum value; andthe technique wherein a determination as to whether at least onepredetermined condition including a condition that the amount ofincrease of the actual pressing force Fs becomes larger than a thirdthreshold value in the first state of the motor 372 is satisfied iseffected to determine whether the operation of the self-servo mechanismis initiated. However, these techniques may be utilized for otherpurposes.

A fourteenth embodiment of this invention will be described next. Thisfourteenth embodiment is structurally similar in many aspects to thetwelfth embodiment of FIGS. 37-39 whose structural arrangement is shownin FIG. 19. With the same reference signs and step numbers as used inthe ninth and twelfth embodiments being used in this fourteenthembodiment, only a difference of the fourteenth embodiment from thetwelfth embodiment will be described in detail.

Referring to FIG. 43, there is shown a motor-driven disc brake 470 usedin an electrically operated braking system for a 4-wheel automotivevehicle, which is constructed according to the fourteenth embodiment ofthe invention. The disc brake 470 is different from the disc brake 310shown in FIG. 19, in that the disc brake 470 uses (a) a DC motor 472 inplace of the ultrasonic motor 372, and (b) a torque transmissionmechanism in the form of a worm gear 474 disposed between the DC motor472 and a motion converting mechanism in the form of the ballscrewmechanism 374. Except for the provision of the DC motor 472 and the wormgear 474, the disc brake 470 is identical with the disc brake 310 shownin FIG. 19.

The caliper 336 carries a drive device 476 on the back side (left sideas seen in FIG. 43) of the inner pad 320 b. The drive device 476 has ahousing 478 at which the drive device 476 is fixed to the caliper 336.The externally threaded member 402 (rotary member of the motionconverting mechanism) of the ballscrew mechanism 374 is supported by thehousing 478 through the radial and thrust bearings 410, 412. Theinternally threaded member 402 is rotatable relative to the housing 478,and an axial movement of the member 402 is restricted by the stop 414.The DC motor 472 and the worm gear 474 are accommodated within thehousing 478.

The worm gear 474 consists of a worm wheel 480 and a worm 482 whichrotatably engage each other, as well known in the art. The axis of theworm wheel 480 and the axis of the worm 482 are perpendicular to eachother. The worm wheel 480 is fixedly mounted on the internally threadedmember 402 such that the worm wheel 480 is coaxial and rotatable withthe member 402. On the other hand, the worm 482 is supported by thehousing 478 such that the worm 482 is rotatable relative to the housing478 while an axial movement of the worm 482 is limited. Radial andthrust loads acting on the worm 482 are received by respective radialand thrust bearings (not shown). To the worm 482, there is fixed arotary shaft of the DC motor 472 such that the worm 482 is coaxial androtatable with the rotary shaft. The axes of the worm 482 and the DCmotor 472 are perpendicular to the plane of the view of FIG. 43.

In the disc brake 470, the DC motor 472 is operated in the forwarddirection with a forward drive signal applied thereto, and the worm 482is rotated in the forward direction, causing the worm wheel 480 and theinternally threaded member 402 to be rotated in the forward direction.The forward rotation of the internally threaded member 402 causes theexternally threaded member 400 (axially movable member of the motionconverting mechanism) to be advanced, so that the presser rod 370 isadvanced to press the pair of friction pads 320 a, 320 b against theopposite friction surfaces 314 of the disc rotor 312.

In the present embodiment, the reverse efficiency of the worm gear 474is set to be zero, so that the torque of the DC motor 472 is transmittedto the internally threaded member 402, but the torque of the internallythreaded member 402 is totally received by the worm gear 474 and is nottransmitted to the DC motor 472. Accordingly, even when the torque ofthe internally threaded member 402 based on the reaction force of theinner pad 320 b due to the self-servo effect is larger than the drivetorque of the DC motor 472, none of the worm wheel 480, worm 482 and DCmotor 472 are rotated in the reverse direction, and these members 480,482, 472 are held in the same angular position. In this arrangement, thepresser rod 370 is held locked resisting the reaction force receivedfrom the inner pad 320 b, and the pressing force acting on the frictionpads 320 is made larger than the drive torque of the DC motor 472, owingto a considerably large self-servo effect of the inner pad 320 b.

While the disc brake 470 is providing the self-servo effect, the wormgear 474 is capable of maintaining the actual pressing force of theinner pad 320 b.

During operation of the brake operating member, the DC motor 472 maygenerate a large amount of heat if the DC motor 472 is held in theenergized state while the rotation of the DC motor 472 is restricted.Further, this condition of the DC motor 472 results in an increase inthe power consumption. In the light of this fact, the present brakingsystem is adapted such that the DC motor 472 is held off while theself-servo effect is provided during activation of the disc brake 470.The DC motor 472 is also held off after the presser rod 370 is advancedto apply the parking brake.

Referring to the block diagram of FIG. 44, there is shown an electriccontrol system for the present electrically operated braking system. Theelectric control system includes a primary brake controller 484 to whichis connected the DC motor 472 through the motor driver circuit 454. Thisprimary brake controller 484 is basically similar to the primary brakecontroller 430 used in the twelfth embodiment, and includes a computeradapted to execute a pad pressing control routine different from theroutine of FIG. 37 of the twelfth embodiment.

The pad pressing control routine according to the fourteenth embodimentof the invention is illustrated in the flow chart of FIG. 45. The samestep numbers as used in FIG. 37 are used in FIG. 45, to identify thesame steps as in FIG. 37.

If the actual pressing force Fs(N) is smaller than the desired value F*,that is, if the affirmative decision (YES) is obtained in step S105, thecontrol flow goes to step S151 in which the self-servo effect monitoringroutine is executed in the same manner as in the twelfth embodiment,namely, as illustrated in the flow chart of FIG. 38. If the self-servoeffect is not provided, that is, if the negative decision (NO) isobtained in step S152, the control flow goes to step S171 in which theforward drive signal is applied to the DC motor 472. If the self-servoeffect is provided and the affirmative decision (YES) is obtained instep S152, the control flow goes to step S172 to turn off the DC motor472. One cycle of execution of the routine of FIG. 45 is terminated withstep S171 or S172.

If the actual pressing force Fs(N) is larger than the desired value F*,the negative decision (NO) is obtained in step S105 and in step S114,and the flow control goes to step S173 in which the reverse drive signalis applied to the DC motor 472. If the actual pressing force Fs(N) isequal to the desired value F*, the negative decision (NO) is obtained instep S104 while the affirmative decision (YES) is obtained in step S114,so that step S174 is implemented to turn off the DC motor 472. One cycleof execution of the routine is terminated with step S173 or S174.

It will be understood from the above description of the presentfourteenth embodiment of the invention that the insufficient increasepreventing means for preventing shortage of the amount of increase ofthe pressing force Fs is constituted by the worm gear 474, load sensor420 and a portion of the primary controller 484 assigned to implementsteps S151, S152, S171 and S172 of FIG. 45. It will also be understoodthat the self-servo effect monitoring means is constituted by a portionof the controller 484 assigned to implement step S151 of FIG. 45(execute the self-servo effect monitoring routine of FIG. 38).

It is further noted that the present embodiment is arranged to determinewhether the self-servo effect is provided or not, and turn off the DCmotor 472 when the self-servo effect is provided, in order to preventthe generation of heat by the DC motor 472 and minimize the unnecessarypower consumption by the DC motor 472. However, this arrangement is notessential. That is, the DC motor 472 may be held on even while theself-servo effect is provided.

Although the transmission or transfer of the reaction force of the innerpad 320 b to the DC motor 472 is prevented by the worm gear 474 evenwhen the reaction force is larger than the drive force of the DC motor472. However, this function of the worm gear 474 may be achieved by asuitable screw mechanism provided as the motion converting mechanism,whose reverse efficiency is substantially zero.

Next, a fifteenth embodiment of the invention will be described. Thisfifteenth embodiment is similar in many aspects to the ninth embodiment,and is different from the ninth embodiment in the elastic controlmechanism, which will be described in detail.

In the ninth embodiment, the elastic control mechanism 340 shown in FIG.21 has an elastic coefficient which is extremely small. The elasticcoefficient is an elastic force of the elastic member 342 divided by anamount of elastic deformation of the elastic member 342. The elasticforce of the elastic member 342 is equal to a load which the elasticmember 342 receives from the inner pad 320 b, while the amount ofelastic deformation of the elastic member 342 is equal to an amount ofdisplacement of the inner pad 320 b which occurs due to the dragging ofthe inner pad 320 b with the disc rotor 312). Since the elasticcoefficient of the elastic control mechanism 340 is extremely small, theamount of elastic deformation of the elastic member 342 suddenlyincreases with a result of a sudden increase of the amount ofdisplacement of the inner pad 320 b due to its dragging with the discrotor 312, after the elastic force of the elastic member 342 hasincreased to the set load or pre-load of the elastic member 342, asindicated by broken line in the graph of FIG. 47. The amount of elasticdeformation is held zero, preventing the dragging of the inner pad 320 bwith the disc rotor 312, until the elastic force has increased to thepre-load value.

Generally, the actual pressing force Fs of the inner pad 320 b tends tobe increased at an excessively high rate owing to the self-servo effect.To prevent the excessively high rate of increase of the actual pressingforce, the pre-load of the elastic control mechanism 340 may be set tobe relatively large, for example. However, while the relatively largepre-load of the elastic control mechanism 340 is effective to restrictthe rate of increase of the actual pressing force, it causes a drawbackthat the dragging of the inner pad 320 b with the disc rotor 320 b isdelayed, whereby the initiation of the self-servo effect of the innerpad 320 b is accordingly delayed. On the other hand, a relatively smallpre-load of the elastic control mechanism 340 permits the self-servoeffect to be initiated at a relatively early point of time, but tends tocause an excessively high rate of increase of the actual pressing force.

In the light of the above analysis, the present fifteenth embodimentuses an elastic control mechanism, which is designed such that theelastic force generated increases with an increase in the amount ofdeformation. For instance, the elastic control mechanism may be designedsuch that the elastic force increases non-linearly with respect to theamount of deformation. Two examples of the non-linear relationship areindicated by solid lines A and B in the graph of FIG. 47. The solid lineA represents a first non-linear relationship wherein the elasticcoefficient changes in two steps. Described more specifically, the firstnon-linear relationship provides a first comparatively low elasticcoefficient while the amount of elastic deformation is comparativelysmall, and a second comparatively high elastic coefficient while theamount of elastic deformation is comparatively large. The solid line Brepresents a second non-linear relationship wherein the elasticcoefficient continuously increases with an increase in the amount ofelastic deformation. The elastic control mechanism may also be designedsuch that the elastic force increases linearly with respect to theamount of deformation. An example of the linear relationship isindicated by a two-dot chain line C in the graph of FIG. 47.

The elastic control mechanism according to the present fifteenthembodiment may be constructed as shown in FIG. 46, in which themechanism is indicated generally at 500. Like the elastic controlmechanism 340, the elastic control mechanism 500 includes (a) a firstelastic member in the form of a U-shaped spring 502 having a pair ofarms 501 a, 501 b, and (b) an adjusting mechanism 504 for adjusting thepre-load of the U-shaped spring 502 by changing the amount ofdeformation of the U-shaped spring 502. The adjusting mechanism 504includes an adjusting bolt 506 which extends in the direction Z in whichthe inner pad 320 b is dragged with the disc rotor 312. The adjustingbolt 506 connects the pair of arms 501 a, 501 b of the U-shaped spring502 such that the two arms 501 are movable toward each other, but notmovable away from each other. The pre-load of the U-shaped spring 502can be adjusted by operating the adjusting bolt 506 to change theinitial amount of elastic deformation of the U-shaped spring 502.

The elastic control mechanism 500 further includes a second elasticmember in the form of a coil spring 508 which is disposed between thetwo arms 501 of the U-shaped spring 502 and through which the adjustingbolt 506 extends coaxially therewith. The length of the coil spring 508in a free state thereof is determined such that there is left a suitableclearance δ between one of the opposite ends of the coil spring 508 andthe inner surface of the corresponding one of the two arms 501 a, 501 b.With a force R being applied from the inner pad 320 b to the arm 501 b(on the side of the inner pad 320 b), only the U-shaped spring 502undergoes elastic deformation while the force R is relatively small,that is, while the amount of dragging movement of the inner pad 320 bwith the disc rotor 312 is relatively small. When the force R hasexceeded a given threshold with an increase in the amount of draggingmovement of the inner pad 320 b, the coil spring 508 begins to undergoelastic deformation, with the continuing elastic deformation of theU-shaped spring 502. Consequently, the elastic control mechanism 500exhibits the first non-linear relationship represented by the solid lineA in FIG. 47.

Various changes or modifications may be made to the elastic controlmechanism 500. For instance, the coil spring 508 may be replaced by aplurality of coned disc springs 512 as shown in FIG. 48, or by a sheetspring 156 as shown in FIG. 49. The sheet spring 156 is fixed at one endthereof to the arm 501 b (movable arm) on the side of the inner pad 320b, such that there is left a suitable clearance δ between the free endof the sheet spring 156 and the outer surface of the arm 501 b.

The elastic control mechanism of FIG. 48 is similar in the principle ofoperation to the elastic control mechanism of FIG. 46, in that only oneof the two elastic members disposed in parallel undergoes elasticdeformation while the amount of deformation is relatively small, andboth of the two elastic members simultaneously undergo elasticdeformation while the amount of elastic deformation is relatively large,so that the elastic coefficient increases when the amount of elasticdeformation exceeds a certain threshold. On the other hand, the elasticcontrol mechanism of FIG. 49 is adapted such that both of the twoelastic members disposed in series simultaneously undergo elasticdeformation while the amount of elastic deformation of the mechanism isrelatively small, and only one of the two elastic members undergoeselastic deformation while the amount of elastic deformation isrelatively large, so that the elastic coefficient increases with theamount of elastic deformation.

Further modifications of the elastic control mechanism 500 may includethe provision of: a cylindrical rubber member disposed radiallyoutwardly of the coil spring 508 or an assembly of the coned discsprings 512; a rubber member or members disposed in a gap or gapscorresponding to the clearance δ between one or both ends of the coilspring 508 or the assembly of the coned disc spring 512 and one or bothof the arms 501 a, 501 b; and a rubber member or members interposedbetween the adjacent turns of the coil spring 508 or between theadjacent coned disc springs 512. These modified arrangements provide thesecond non-linear relationship represented by the solid line B in FIG.47. Where the rubber member or members is/are interposed between theadjacent turns of the coil spring 508, the clearances between theadjacent turns may or may not be increased from the nominal value. Inother words, the coil spring 508 may be provided with a relatively largeamount of clearances between the adjacent turns, for the purpose ofproviding the rubber member or members in the clearances. Where therubber member or members is/are provided between the adjacent coned discsprings 512, the rubber members and the coned disc springs 512 arealternately superposed on each other.

It will be understood from the above description of the fifteenthembodiment of the invention that the elastic control mechanism 500functions as an elastic member and constitutes a mechanism forcontrolling the rate of increase of the pressing force of the inner pad320 b.

A sixteenth embodiment of this invention will now be explained. Thissixteenth embodiment is similar in many aspects to the third embodimentof FIGS. 10-12, but is different from the third embodiment only in somepart of the disc brake. The same reference signs as used in FIGS. 10-12will be used in this sixteenth embodiment to identify the commonly usedelements, and only this part of the disc brake of the present embodimentwill be described.

In the disc brake 150 of the third embodiment of FIGS. 10-12, the innerpad 14 b functions as a wedge to provide the self-servo effect. In adisc brake 520 according to the present sixteenth embodiment, on theother hand, the outer pad 14 a functions as a wedge to provide theself-servo effect, as is apparent from FIG. 50.

In the disc brake 520, the inner pad 14 b is not supported by themounting bracket 152 so as to positively allow the inner pad 14 b to bedragged along with the disc rotor 11. In the inner pad 14 b, both thefriction member 18 and the backing plate 20 have a constant thickness.The presser rod 216 and the ultrasonic motor 212 are disposed in thedisc brake 520 such that the axes of the presser rod 216 and the motor212 are perpendicular to the friction surfaces 12 of the disc rotor 11.

On the other hand, the outer pad 14 a is supported by the mountingbracket 152 so as to positively allow the outer pad 14 a to be draggedwith the disc rotor 11. In the outer pad 14 a, the friction member 18has a constant thickness while the backing plate 20 has a thicknesswhich continuously decreases in the dragging direction Y. Namely, thebacking plate 20 has a slant back surface 524 which is inclined withrespect to the friction surfaces 12 of the disc rotor 11. Like the discbrake 150 of the third embodiment, the disc brake 520 has the elasticmember 184, movable member 186 and stop 190 for controlling the draggingmovement of the outer pad 14 a with the disc rotor 11. The reactionportion 206 of the caliper 202 has a support surface 526 for supportingthe backing plate 20 of the outer pad 14 a at its slant surface 524 suchthat the outer pad 14 a can be moved due to its friction contact withthe disc rotor 11. This support surface 526 is also inclined withrespect to the friction surfaces 12 of the disc rotor 11. Between thesupport surface 526 and the outer pad 14 a, there is provided frictionreducing means in the form of a thrust bearing 528, which includes aplurality of rolling members in the form of balls arranged along acircle.

Various modifications may be made to the sixteenth embodiment of theinvention. For instance, the inner pad 14 b is also provided with aslant surface 530 as indicated in FIG. 51. In this case, however, theinner pad 14 b does not function as a wedge.

Next, a seventeenth embodiment of this invention will be described. Thisembodiment is similar in many aspects to the sixteenth embodiment, andis different from the sixteenth embodiment only in the self-servo effectinhibiting mechanism. The same reference signs as used in the sixteenthembodiment will be used in this seventeenth embodiment to identify thecommonly used elements, and only the self-servo effect inhibitingmechanism.

In the conventional disc brake, the mounting bracket generally includes(a) a pair of portions between which the outer pad is disposed in therotating direction of the disc rotor, and (b) a bridging portion whichextends behind the outer pad so as to connect the pair of portions. InFIG. 20, the pair of portions are indicated at 538 a, 538 b, and thebridging portion is indicated at 540.

In the present seventeenth embodiment, the bridging portion 540 isreplaced by an elastic member 542, as shown in FIGS. 52 and 53. Theelastic member 542 is a generally rod-shaped member having opposite endportions 543 a, 543 b, while the outer pad 14 a has opposite endportions 544 a, 544 b which are opposite to each other in the rotatingdirection X of the disc rotor. During forward running of the vehicle,the outer pad 14 a is dragged with the disc rotor in the direction fromthe end portion 544 a toward the end portion 544 b. The end portion 543b of the elastic member 542 is associated with the end portion 544 b ofthe outer pad 14 a, while the end portion 543 a is associated with theportion 538 a of the mounting bracket 330 which is remote from the endportion 544 b of the outer pad 14 a.

The end portion 543 b of the elastic member 542 engages a surface of theend portion 544 b of the outer pad 14 a, which surface faces in thedragging direction of the outer pad 14 a. The end portion 543 b receivesa force from that surface of the end portion 544 b, which force acts inthe dragging direction. The elastic member 542 undergoes elasticdeformation due to this force, so that the elastic property of theelastic member 542 is optimized so as to suitably determine the point ofinitiation of the dragging movement of the outer pad 14 a, namely, thepoint of initiation of the self-servo effect of the outer pad 14 a.

Referring to FIG. 54 showing the end portions 543 b and 544 b inenlargement, the portion 538 b of the mounting bracket 152 is formedwith a stop 546 which limits an amount of displacement of the endportion 543 b of the elastic member 542 toward the end portion 544 b ofthe outer pad 14 a. This stop 546 is effective to establish apredetermined initial amount of clearance between the end portion 543 bof the elastic member 542 and the end portion 544 b of the outer pad 14a. The end portion 543 b and the portion 538 b are held in contact witheach other through a support 548. That is, the support 548 preventsdirect contact of the end portions 543 b and the portion 538 b when theouter pad 14 a is dragged due to frictional contact with the disc rotor.

The elastic member 542 has an elastic property similar to the non-linearrelationship as described above with respect to the fifteenthembodiment. The end portion 543 b of the elastic member 542 has a cutout550 as shown in FIG. 52. When the load or force transmitted from theouter pad 14 a to the end portion 543 b is small enough for the cutout550 to exist, the elastic member 542 has a small minimum modulus ofsection and exhibits a relatively small elastic coefficient. After theabove-indicated load or force has increased to an extent that causes thecutout 550 to disappear, the minimum modulus of section of the elasticmember 542 is increased, and the elastic coefficient is accordinglyincreased. In this arrangement, the elastic member 542 having the cutout550 exhibits a non-linear relationship based on a difference in itsminimum modulus of section when the cutout 550 exists and when thecutout 550 does not exist.

As shown in FIG. 54, there exists a gap between the end portion 543 b ofthe elastic member 542 and the portion 538 b of the mounting bracket152. The amount of this gap decreases as the outer pad 14 a is draggedwith the disc rotor. The gap may be filled with a second elastic member552 made of a rubber material. In this case, the elastic member 542cooperates with the second elastic member 552 to constitute the elasticmember whose elastic coefficient continuously varies so as to provide anon-linear relationship between the elastic force and the amount ofelastic deformation.

The end portion 543 a of the elastic member 542 is fixed by a bolt 554to the portion 538 a of the mounting bracket 142. However, the fixing ofthe end portion 543 a to the portion 538 a by the bolt 554 is notessential. The end portion 543 a and the portion 538 a may be associatedwith each other such that a relative movement of the end portion 543 aand the portion 538 a in a first direction is prevented by a firststructure in which a protrusion engages a groove, while a relativemovement of the end portion 543 a and the portion 538 a in a seconddirection perpendicular to the first direction is prevented by a secondstructure in which a pin engages a hole. Examiners of the first andsecond structures are illustrated in FIG. 56, wherein the end portion543 a has a protrusion 556 which engages a groove 558 formed in theportion 538 a and which cooperates with the protrusion 556 to constitutethe first structure indicated above. Further, the end portion 543 a andthe portion 538 a have respective holes 560, 562 aligned with eachother, and a pin 564 is inserted through these two holes 560, 562. Thepin 564 and the holes 560, 562 constitute the second structure indicatedabove.

While the mounting bracket 152 of the disc brake according to thepresent seventeenth embodiment does not have the bridging portion 540,the mounting bracket 152 may have the bridging portion 540 disposedadjacent and parallel to the elastic member 542 described above. The pin564 may be replaced by a bolt or screw.

In the embodiment of FIG. 52, the cutout 550 is formed in the outersurface of the elastic member 542, at a junction between the end portion543 b and an intermediate portion 566 of the elastic member 542 whichare connected to each other at right angles. The cutout 550 has a depthcorresponding to a half of the thickness of the elastic member 542 asseen in FIG. 52. However, the cutout 550 may replaced by a cutout 568which is formed in the end portion 543 b so as to extend toward the discrotor, as shown in FIG. 56. Further, a second elastic member 570 may beattached to the end portion 543 b of the elastic member 542, such thatthe force received from the outer pad 14 a due to its dragging with thedisc rotor acts on only the elastic member 542 and does not act on thesecond elastic member 570 while the force is relatively small, and actson both of the elastic member 542 and the second elastic member 570while the force exceeds a certain limit.

It will be understood that the elastic member 542 functions as theelastic member, which acts as the mechanism for controlling the rate ofincrease of the pressing force of the outer pad 14 a.

There will be described an eighteenth embodiment of this invention,which is similar in many aspects to the sixteenth embodiment. The samereference signs as used in the sixteenth embodiment will be used in thepresent eighteenth embodiment to identify the common used elements.

Only a difference of the eighteenth embodiment from the sixteenthembodiment will be used.

In the sixteenth embodiment, the backing plate 30 of the outer pad 14 ahas the slant back surface 524, as shown in FIG. 50. The slant backsurface 524 lies in a single plane and has a constant angle ofinclination with respect to the friction surface 12 of the disc rotor11. In the present eighteenth embodiment, the backing plate 20 of theouter pad 14 a has a slant back surface 572 as shown in FIG. 58. Thereaction portion 206 has a support surface 574 which has apart-spherical projection for contact with the slant back surface 572 ofthe backing plate 20.

The outer pad 14 a provided in the eighteen embodiment is shown inenlargement in FIG. 59. As shown in this figure. the slant back surface572 of the backing plate 20 of the outer pad 14 a has a first slightlyinclined part 576, and a second inclined part 578 whose angle ofinclination is larger than that of the first slightly inclined part 576.These first and second inclined parts 576, 578 are arranged in thisorder in a direction opposite to the dragging direction of the outer pad14 a. As the outer pad 14 a is dragged due to its frictional contactwith the disc rotor 11, the part-spherical projection of the supportsurface 574 of the reaction portion 206 first contacts the firstslightly inclined part 576 and then comes into contact with the secondinclined part 578. The first and second inclined parts 576, 578 lie inrespective two planes which are not parallel to each other.

Before the outer pad 14 a provides the self-servo effect, the supportsurface 574 of the reaction portion 206 contacts the first inclined part576, which has a relatively small angle of inclination and thereforepermits easy dragging movement of the outer pad 14 a with the disc rotor11. In other words, the first inclined part 576 of the slant backsurface 572 of the backing plate 20 permits easy initiation of theself-servo effect. After the initiation of the self-servo effect, thesupport surface 574 contacts the second inclined part 578 whose angle ofinclination is large enough for the self-servo effect to be sufficient.

It will be understood that the slant surface 572 of the backing plate 20of the outer pad 14 a functions as the mechanism for controlling therate of increase of the pressing force of the outer pad 14 a.

Referring to FIGS. 60-62, there will be described nineteenth, twentiethand twenty-first embodiments of this invention, which are identical withthe eighteenth embodiment except for the slant surface of the outer pad14 a which will be described.

In the nineteenth embodiment of FIG. 60, the backing plate 20 of theouter pad 14 a has a concave slant surface 580. That is, the backsurface 580 of the backing plate 20 follows a circular arc as seen incross section in a plane parallel to the plane of FIG. 60 which isparallel to the dragging direction Y. Thus, the concave slant surface580 has a first inclined part whose angle of inclination is relativelysmall, and a second inclined part whose angle of inclination isrelatively large. Before the self-servo effect is provided, the supportsurface 574 of the reaction portion 206 contacts the first inclinedpart. After the self-servo effect is initiated, the support surface 574contacts the second inclined part. Accordingly, the outer pad 14 a ofthe nineteenth embodiment has substantially the same advantage as thatof the eighteenth embodiment of FIG. 59. The concave slant surface 580functions as the mechanism for controlling the rate of increase of thepressing force of the outer pad 14 a.

In the twentieth embodiment of FIG. 61, the backing plate 20 of theouter pad 14 a has a slant back surface 582 consisting of a firstinclined part 584 whose angle of inclination is relatively small, asecond inclined part 586 whose angle of inclination is larger than thatof the first inclined part 584, and a third inclined part 588 whoseangle of inclination is smaller than that of the second inclined part586. These three inclined parts 584, 586, 588 are arranged in this orderin the direction opposite to the dragging direction of the outer pad 14a, and lie in respective three planes. In this arrangement, the supportsurface 574 of the reaction portion 306 contacts the first inclined part548 before the self-servo effect is provided. The relatively small angleof inclination of the first inclined part 584 permits easy initiation ofthe self-servo effect. After the initiation of the self-servo effect,the support surface 574 contacts the second inclined part 586 whoseangle of inclination is large enough for the self-servo effect to besufficient. Before the self-servo effect becomes excessively large, thesupport surface 574 comes into contact with the third inclined part 588whose angle of inclination is smaller than that of the second inclinedpart 586, so that the excessive self-servo effect is prevented. Theslant back surface 584 functions as the mechanism for controlling therate of increase of the pressing force of the outer pad 14 a.

In the twenty-first embodiment shown in FIG. 62, the backing plate 20 ofthe outer pad 14 a has a slant back surface 590 consisting of a concavefirst inclined part 592 and a convex second inclined part 594, which arearranged in this order in the direction opposite to the draggingdirection Y of the outer pad 14 a. The first or concave part 594consists of a first section whose angle of inclination is relativelysmall, and a second section whose angle of inclination is larger thanthat of the first section. Before the self-servo effect is provided, thesupport surface 574 of the reaction portion 206 contacts the firstsection of the concave part 592. The relatively small angle ofinclination of the first section permits easy initiation of theself-servo effect. After the initiation of the self-servo effect, thesupport surface 574 comes into contact with the second section of theconvex part 594 whose angle of inclination is large enough for theself-servo effect to be sufficient. Before the self-servo effect becomesexcessively large, the support surface 574 comes into contact with thesecond or convex part 594 whose angle of inclination is smaller thanthat of the concave part 592, so that the excessive self-servo effect isprevented. The slant back surface 590 functions as the mechanism forcontrolling the rate of increase of the pressing force of the outer pad14 a.

A twenty-second embodiment of the present invention will be described.This embodiment is similar in many aspect to the first embodiment. Thesame reference signs as used in the first embodiment will be used inthis twenty-second embodiment, and only a difference of thetwenty-second embodiment from the first embodiment will be described.

In the first embodiment, the drive force of the ultrasonic motor 72 isboosted by the pair of levers 30, 30, and the boosted force istransmitted to the pair of friction pads 14, 14 to force these frictionpads 14 onto the disc rotor 11, so that the braking force applied to thevehicle wheel in question is considerably larger than the drive force asproduced the ultrasonic motor 72. However, the boosting of the driveforce of the motor 72 can be achieved without the self-servo effectbeing provided by levers.

In the light of the above, the twenty-second embodiment shown in FIG. 63is adapted such that a pair of levers 600 are supported by a mountingmember 602 through respective pins 604 such that the levers 600 arepivotable about respective axes which are perpendicular to the axis ofrotation of the disc rotor 11. Each lever 600 is adapted to boost thedrive force of the ultrasonic motor 72 according to its lever ratio, sothat the boosted drive force acts on the corresponding friction pad 14at its backing plate 20. The mounting member 602 functions not only as asupport member for pivotally supporting the levers 600, but also as asupport member for supporting the pair of friction pads 14 such that thefriction pads 14 are slidably movable toward and away from the discrotor 11, and as a member for receiving the friction force from eachfriction pad 14.

As in the first embodiment, the ultrasonic motor 72 is controlled in afeedback fashion by the controller 100, on the basis of the outputsignals received from the depression force sensor 102 and the brakingforce sensor 110, so that the braking force acting on the vehicle wheelin question is controlled depending upon the operating force acting onthe brake pedal.

In the embodiment of FIG. 63, the drive force of the ultrasonic motor 72is transmitted to the friction pads 14 through a simple mechanismincluding the levers 600 as major elements. The disc brake is capable ofproducing a wheel braking force considerably larger than the drive forceof the motor 72, without suffering from complicated construction.

Then, a twenty-third embodiment of this invention will be described byreference to FIG. 64.

The electrically operated braking system of this embodiment of FIG. 64is identical with that of the twenty-second embodiment of FIG. 63,except for the provision of the cooling device 232 used in the fourthembodiment of FIGS. 13-14. The same reference signs as used in FIGS. 13,14 and 64 are used in FIG. 64 to identify the commonly used elements. Inthe present braking system wherein the drive force produced by theultrasonic motor 72 and boosted by the levers 600 is transmitted to thefriction pads 14, the ultrasonic motor 72 is positively cooled by thecooling device 232, to assure improved operating stability of the motor72 with a reduced adverse thermal influence.

There will next be described a twenty-fourth embodiment of thisinvention, which is similar in many aspects to the ninth embodiment.

Referring to FIG. 65, there is shown an electrically operated brakingsystem constructed according to the twenty-fourth embodiment, whichincludes a motor-driven disc brake 710.

The motor-drive disc brake 710 has a disc rotor 712 functioning as arotary member which is rotated with a vehicle wheel to be braked. Thedisc rotor 712 has opposite friction surfaces 714, while the disc brake710 includes a pair of friction pads 720 a, 720 b disposed opposite tothe respective friction surfaces 714 of the disc rotor 712. Each ofthese two friction pads 720 has a friction member 722, and a backingplate 724 which is fixed to the back surface of the friction member 722and which is made of a steel material.

The disc brake 710 includes a pad support mechanism 726, a self-servomechanism 727, and a pad presser mechanism 728.

The pad support mechanism 726 will be described first.

As shown in FIG. 66, the disc brake 710 is provided with a mountingbracket 730 which is fixed to the body of the vehicle, in a cantileverfashion, so as to extend over the periphery of the disc rotor 712. Themounting bracket 730 includes (a) portions which are located on theopposite sides of the disc rotor 712 and which support the respectivefriction pads 720 a, 720 b such that the friction pads 720 are movablein a direction intersecting the friction surfaces 714, and (b) portionsfunctioning as a bearing member, which portions receive friction forcesgenerated due to frictional contacts of the friction pads 720 with thefriction surfaces 714 of the disc rotor 712. In FIG. 66, “X” representsa direction of rotation of the disc rotor 712 during forward running ofthe vehicle, while “Y” represents a direction in which each of thefriction pads 720 is movable relative to the friction surfaces 714. Thedirection Y is perpendicular to the friction surfaces 714. The mountingbracket 730 is fixed to the vehicle body such that the upper portion ofthe mounting bracket 730 as seen in FIG. 66 is located on the front sideof the vehicle while the right and left portions of the mounting bracket730 as seen in FIG. 66 are located on the outer and inner sides of thevehicle as viewed in the lateral or transverse direction of the vehicle.Therefore, the friction pad 720 a on the right side of the vehicle isreferred to as an outer pad while the friction pad 720 b on the leftside is referred to as an inner pad.

Then, the self-servo mechanism 727 will be described.

The self-servo mechanism 727 is adapted to enable the inner pad 720 b tofunction as a wedge which provides a self-servo effect. To this end, theinner pad 720 b is supported by the mounting bracket 730 such that theinner pad 720 b is positively allowed to be dragged along with the discrotor 712 due to frictional contact of the inner pad 720 b with the discrotor 712. The structure of the mounting bracket 730 for supporting theinner pad 720 b in this manner is similar to that in the thirdembodiment of FIGS. 10-12. In FIG. 66, “Z” represents a direction inwhich the inner pad 720 b is dragged with the disc rotor 712 during theforward running of the vehicle. The inner pad 720 b is wedge-shaped withthe thickness of the friction member 722 continuously decreasing in thedragging direction “Z”, namely, in the direction from the rear sidetoward the front side of the vehicle. Thus, the friction member 722 ofthe inner pad 720 b has a slant surface 734 which is inclined withrespect to the opposite surfaces of the backing plate 724 and which isopposed to the friction surface 714 of the disc rotor 712. With theslant surface 734 held in contact with the friction surface 714, theback surface of the backing plate 724 remote from the friction member722 is inclined with respect to the friction surface 714. Thus, thebacking plate 724 is inclined with respect to the friction surface 714.For a presser rod (which will be described) to engage the backing plate724 such that the axis of the presser rod is perpendicular to the backsurface of the backing plate 724, the mounting bracket 730 is fixed tothe vehicle body such that a reference line L1 of the mounting bracket730 is inclined with respect to an axis L2 of rotation of the disc rotor712 so that the left portion of the mounting bracket 730 as seen in FIG.66 is displaced toward the front portion of the vehicle. The referenceline L1 is a straight line which passes the centers of the friction pads720 a, 7320 b and is parallel to the direction Y in which the pads 720are movable. The reference line L1 is also parallel to the direction inwhich a caliper 736 engageable with the backing plates 724 of thefriction pads 720 is slidably movable relative to the mounting bracket730 to which the caliper 736 is slidably attached through pins.

The outer pad 720 a is not intended to provide a self-servo effect. Inthis sense, the outer pad 720 a need not be wedge-shaped. However, theouter pad 720 a is also wedge-shaped following the angle of inclinationof the caliper 726 whose direction of movement is parallel to thereference line L1 of the mounting bracket 730 which is inclined withrespect to the rotation axis L2 of the disc rotor 712 by the angle ofinclination of the backing plate 724 of the inner pad 720 b with respectto the friction surfaces 714 of the disc rotor 712. Unlike the frictionmember 722 of the inner pad 720 b, the friction member 722 of the outerpad 720 a has a thickness which continuously increases in the draggingdirection Z of the inner pad 720 b or in the rotating direction X of thedisc rotor 712. The wedge shape of the outer pad 720 a permits itsfriction member 722 to contact the friction surface 714 of the discrotor 712 without a gap or clearance therebetween over the entire areaof the friction surface 714.

As described above, the mounting bracket 730 supports the inner pad 720b so as to positively allow the inner pad 720 b to be moved or draggedwith the disc rotor 712 due to the frictional contact therebetween.However, the mounting bracket 730 supports the outer pad 720 b so as tosubstantially inhibit the outer pad 720 a from being moved with the discrotor 712.

The inner pad 720 b is not always allowed to be dragged with the discrotor 712. Namely, the inner pad 720 b is supported such that thedragging movement of the inner pad 720 b with the disc rotor 712 ispermitted only after the friction force acting on the inner pad 720 bexceeds a predetermined threshold. Described more specifically, theinner pad 720 b is associated with the mounting bracket 730 via anelastic control mechanism 740. The elastic control mechanism 740 has anelastic member which receives a load from the inner pad 720 b. Theelastic member is not elastically deformed until the received load issmaller than the predetermined threshold, so that the inner pad 720 b isinhibited from being moved relative to the mounting bracket 730 in thedragging direction Z, that is, inhibited from being moved with the discrotor 712, until the load acting on the elastic member is smaller thanthe threshold. After the load exceeds the threshold, the elastic memberof the elastic control mechanism 740 is elastically deformed, allowingthe inner pad 720 b to be moved relative to the mounting bracket 730 anddragged with the disc rotor 712.

The elastic control mechanism 740 provided in the present embodimentincludes (a) a U-shaped elastic member 742 having a pair of arms, and(b) an adjusting mechanism 744 for changing an initial amount of elasticdeformation of the elastic member 742, to thereby adjust a pre-loadacting on the elastic member 742. This pre-load is equal to theabove-indicated predetermined threshold above which the inner pad 720 bis permitted to be moved in the dragging direction Z against the biasingaction of the elastic member 742. The elastic member 742 is positionedsuch that the pair of arms extend in the lateral or transverse directionof the vehicle. One of the arms is secured to the mounting bracket 730while the other arm is fixed to the inner pad 720 b. The adjustingmechanism 744 includes an adjusting bolt which extends in a directionsubstantially parallel to the dragging direction Z and which connectsthe two arms of the elastic member 742 so as to permit movements of thetwo arms toward each other and inhibit movements of the two arms awayfrom each other. The adjusting bolt permits adjustment of the spacingdistance between the two arms to thereby permit adjustment of thepre-load acting on the elastic member 742.

In this twenty-fourth embodiment, the predetermined threshold of thefriction force of the inner pad 720 b, or the pre-load of the elasticcontrol mechanism 740 is equal to the friction force which is generatedbetween the disc rotor 712 and the inner pad 720 b when the decelerationof the vehicle achieved by activation of the disc brake 710 is about0.5-0.6 G. When the deceleration of the vehicle is lower than thisthreshold of about 0.5-0.6 G with the brake pedal being operated in anordinary or normal manner, the elastic control mechanism inhibits thedragging of the inner pad 720 b with the disc rotor 712 to therebyinhibit a self-servo effect of the inner pad 720 b. When the vehicledeceleration exceeds the threshold with the brake pedal being abruptlydepressed by a relatively large amount, the elastic control mechanismallows the inner pad 720 b to be dragged with the disc rotor 712,permitting the inner pad to achieve the self-servo effect.

The pad presser mechanism 728 will then be explained.

The disc brake 710 includes the caliper 736 shown in FIG. 65 and 66. Asshown in FIG. 65, the caliper 736 has a body portion 761, and a motorhousing 780 which will be described. The body portion 761 includes a padpresser portion 736 a, a motor mounting portion 736 b and a supportportion 736 c, which are integral with each other. The body portion 761and the motor housing 780 are bolted together. As shown in FIG. 66, thebody portion 761 further includes a pair of arms 762 extending in thelongitudinal direction of the vehicle, as shown in FIG. 66. The arms 762are also formed integrally with the body portion 761.

The body portion 761 of the caliper 736 is supported at the pad presserportion 736 a by the mounting bracket 730 such that the body portion 761is slidably movable in the direction Y in which the friction pads 720are movably supported by the mounting bracket 730. It will be understoodthat the caliper 736 is a floating caliper. The two arms 762 areconnected at their end portions to respective two pins 763 which extendin the direction Y. These two pins 763 engage the mounting bracket 730such that the pins 763 are slidable in the direction Y. Thus, the bodyportion 761 are slidably supported by the mounting bracket 730, at thepad presser portion 736 a and through the two pins 763.

The pad presser portion 736 a of the body portion 761 of the caliper 736consists of (a) a presser portion 764 disposed adjacent to the backingplate 724 of the inner pad 720 b, (b) a reaction portion 766 disposedadjacent to the backing plate 724 of the outer pad 720 a, and (c) aconnecting portion 768 which extend over the periphery of the disc rotor712 so as to connect the presser and reaction portions 764, 766.

As shown in FIG. 65, a presser rod 770 slidably engages the presserportion 764, such that the front end face of the presser rod 770 facesthe backing plate 724 of the inner pad 720 b, for abutting contact withthe back surface of this backing plate 724. The caliper 736 serves as apresser member for pressing the outer pad 720 a. On the back side of thepresser rod 770, a ultrasonic motor 772 is disposed coaxially with thepresser rod 770. The presser rod 370 and the ultrasonic motor 372 aredisposed such that their axes are parallel to the direction Y. Further,the presser rod 370 and the ultrasonic motor 372 are operatively andcoaxially connected to each other through a ballscrew mechanism 774. Acommon axis L3 of the presser rod 770, ultrasonic motor 772 andballscrew mechanism 774 is parallel to the reference line L1 of themounting bracket 730, and is offset by a suitable distance from thereference line L1 in the rotating direction X of the disc rotor 712, asindicated in FIG. 66.

It will be understood from the above description of the twenty-fourthembodiment that the inner pad 720 b is interposed between the disc rotor712 and the presser rod 770 such that the inner pad 720 b can be movedwith the disc rotor 712 due to the frictional contact of the slantsurface 734 with the friction surface 714, with the presser rod 770 heldin abutting contact with the backing plate 724 of the inner pad 720 b.When the inner pad 720 b is moved with the disc rotor 712, the inner pad720 b functions as a wedge, and the friction force generated between theinner pad 720 b and the disc rotor 712 is converted into an axial forcewhich acts on the disc rotor 712 and the presser rod 770 in oppositedirections so as to move the presser rod 770 away from the disc rotor712. Accordingly, the force by which the friction pads 720 are pressedagainst the opposite friction surfaces 714 of the disc rotor 712 isincreased, whereby the friction force between the inner pad 720 b andthe disc rotor 712 is increased. Thus, the dragging movement of theinner pad 720 b with the disc rotor 712 causes the self-servo effect.

The ultrasonic motor 772 is of a travelling-wave type. As well known inthe art, the motor 772 has a stator 782 and a rotor 784 which arecoaxially disposed within a motor housing 780, as shown in FIG. 65. Inoperation, the stator 782 produces a surface wave upon application of aultrasonic vibration thereto, and the rotor 784 is rotated with afriction force acting between the stator 782 and the rotor 784.

The motor housing 780 consists of a body portion 780 a and a closureportion 780 b which closes a through-hole formed through the bottom wallof the body portion 780 a. These body portion 780 a and the closureportion 780 b are initially separate members which are screwed to eachother. The motor housing 780 is screwed at its open end to the motormounting portion 736 b of the caliper 736.

The stator 782 consists of an elastic body 790 and a piezoelectric body792 both of which take the form of a ring. The elastic and piezoelectricbodies 790, 792 are superposed on each other and bonded together.

The rotor 784 is forced by a pressing contactor mechanism 794 onto thestator 382, so that there is produced a suitable amount of frictionforce therebetween. The rotor 784 has a friction member bonded theretofor frictional contact with the stator 782, so that a travelling-wavevibration generated by the stator 782 is transmitted to the rotor 784,whereby the rotor 784 is rotated. A certain friction force existsbetween the stator 782 and the rotor 784 even when the piezoelectricbody 792 is in a de-energized or off state without a voltage applicationthereto by the pressing contactor mechanism 794. In the presentembodiment, the pressing contactor mechanism 794 is principallyconstituted by a coned disc spring 796. However, the coned disc spring796 may be replaced by a coil spring.

The ballscrew mechanism 774 indicated above includes an externallythreaded member (threaded shaft) 800, an internally threaded member(nut) 802, and a plurality of balls through which the externally andinternally threaded members 800, 802 engage each other. The externallythreaded member 800 is not rotatable but is axially movable while theinternally threaded member 802 is rotatable but is not axially movable.In the present embodiment, the externally threaded member 800 functionsas a movable member, while the internally threaded member 802 functionsas a rotatable member.

The externally threaded member 800 has a splined portion 803 splined tothe motor housing 780 such that the member 800 is not rotatable relativeto the motor housing 780. The splined portion 803 is fixed to the motorhousing 780.

To the internally threaded member 802, there are fixed the rotor 784 andthe pressing contactor mechanism 794 such that the rotor 784 and themechanism 794 are not rotatable relative to the motor housing 780. Inthis arrangement, forward rotation of the internally threaded member 802by forward rotation of the rotor 784 will cause the externally threadedmember 800 to move in the right direction as seen in FIG. 65, pushingthe presser rod 770 to be advanced for pressing the friction pad 720 bto move toward the disc rotor 712. Conversely, reverse rotation of theinternally threaded member 702 by reverse rotation of the rotor 784 willcause the externally threaded member 800 to move in the left directionas seen in FIG. 65, permitting the presser rod 770 to be retracted andthereby permitting the friction pad 720 b to be retracted away from thedisc rotor 712.

The internally threaded member 802 has a front portion 804 located onone side of the rotor 784 nearer to the inner pad 720 b, and a rearportion 806 on the other side of the rotor 784 remote from the inner pad720 b. The front portion 804 is supported by the above-indicated supportportion 736 c of the caliper 736, which is located at the boundarybetween the body portion 761 and the motor housing 780. Thus, the frontportion 804 extends between the body portion 761 of the caliper 736 andthe motor housing 780. A part of the front portion 804 which correspondsto the body portion 761 is stepped having a large-diameter shaft portion810 nearer to the inner pad 720 b, and a small-diameter shaft portion812 remote from the inner pad 720 b. The body portion 761 has a steppedpart in which the front portion 804 is fitted. This stepped part has alarge-diameter hole 816 corresponding to the large-diameter shaftportion 810, and a small-diameter hole corresponding to thesmall-diameter shaft portion 812.

The internally threaded member 802 is rotatably supported by the supportportion 736 c of the body portion 761 through a radial bearing 820 and aradial thrust bearing 822, which are spaced apart from each other in theaxial direction. The radial bearing 820 is adapted to receive a radialload acting on the internally threaded member 802. The radial bearing820 includes an outer ring an inner ring, which are rotatable relativeto each other through a plurality of rolling elements. On the otherhand, the radial thrust bearing 822 is adapted to receive both radialand thrust loads acting on the internally threaded member 802. Theradial thrust bearing 822 includes a plurality of cups which arerotatable relative to each other through a plurality of rollingelements. The radial thrust bearing 822 may be a tapered roller bearingor a self-aligning roller bearing.

Described more specifically, the radial bearing 820 and the radialthrust bearing 822 are mounted on the front portion 804 of theinternally threaded member 802 such that the radial bearing 820 islocated on the side of the rotor 784 while the radial thrust bearing 822is located on the side of the inner pad 820 b. The radial bearing 820 isinterposed between the internally threaded member 802 and the supportportion 736 c such that the outer ring is fixedly fitted on the surfaceof the small-diameter hole 928 while the inner ring is fixedly fitted onthe surface of the small-diameter shaft portion 812. On the other hand,the radial thrust bearing 822 is interposed between the internallythreaded member 802 and the support portion 736 c such that the cup ofthe bearing 822 nearest to the inner pad 720 b is fixed to an annularshoulder surface 826 of the front portion 804, which is located betweenthe large-diameter and small-diameter shaft portions 810, 812 and whichfaces in the rear direction toward the rotor 784. Further, the cup ofthe radial thrust bearing 822 nearest to the rotor 784 is fixed to anannular shoulder surface 826 of the body portion 761 of the caliper 736,which is located between the large-diameter and small-diameter holes816, 918 and which faces in the front direction toward the inner pad 720b.

The internally threaded member 802 has a groove formed in its outercircumferential surface, and fixing means in the form of a retainer ring834 is fixed in this groove. Between the retainer ring 834 and thelarge-diameter shaft portion 810, there are sandwiched the radialbearing 820, motor mounting portion 736 b and radial thrust bearing 822,so as to substantially prevent an axial movement of the internallythreaded member 802. However, the disc brake 710 may be designed so asto allow the axial movement of the internally threaded member 802 over apredetermined distance, so that the amount of gap between the stator 782and the rotor 784 is larger when the disc brake 710 is not operated thanwhen the disc brake 710 is operated. In this arrangement, the force bywhich the stator 782 is pressed onto the rotor 784 is made comparativelysmall, facilitating the oscillation of the stator 782 upon operation ofthe disc brake 710, to thereby permit a smooth rise of the drive torqueof the motor 772.

The externally threaded member 800 is provided on its end face with aload sensor 840 concentrically attached thereto. The externally threadedmember 800 is adapted to abut on the back surface of the presser rod 770through the load sensor 720, so that the force by which the inner pad720 b is pressed by the motor 772 through the ballscrew mechanism 774can be detected based on the output signal of the load sensor 840.

Referring to the block diagram of FIG. 67, there is shown an electriccontrol system of the present electrically operated braking systemincluding the motor-driven disc brake 710. The control system includes acontroller 850 arranged to control the motor-driven disc brake 710, morespecifically, control the ultrasonic motor 772 for regulating thepressing force by which the inner pad 720 b is pressed by the motor 772.The controller 850 is principally constituted by a computerincorporating a CPU, a ROM and a RAM.

To the input interface of the controller 850, there is connected a brakeoperating force sensor 852 for detecting an operating force or amount ofthe brake operating member in the form of a brake pedal operated by theoperator of the vehicle. The brake pedal is operatively connected to abrake operating device adapted to generate a brake operating forceaccording to the operation of the brake pedal. The output signal of thebrake operating force sensor 852 represents this brake operating force.The load sensor 840 indicated above is also connected to the inputinterface of the controller 850. To the output interface of thecontroller 850 is the ultrasonic motor 772 through a motor drivercircuit (not shown).

The controller 850 is adapted to execute a brake control routineillustrated in the flow chart of FIG. 68, according to a program storedin the ROM.

When the brake pedal is depressed, the brake control routine of FIG. 68is executed to control the ultrasonic motor 772 so that the actualpressing force Fs of the inner pad 720 b of the disc brake 710 for eachwheel of the vehicle coincides with the desired value F*. In the presentembodiment, the ratio of the total front braking force to the total rearbraking force is suitably determined, and the braking force for eachwheel is determined for braking the vehicle so as to brake achieve thedesired deceleration value of the vehicle and so as to prevent lockingof the rear wheels prior to that of the front wheels.

The brake control routines of FIG. 68 are executed sequentially for thefour wheels, and the routine for each wheel is repeatedly executed witha predetermined cycle time T.

The brake control routine is initiated with step S601 in which apressing force signal indicative of the brake operating force f isreceived from the brake operating force sensor 852. Step S601 isfollowed by step S602 to calculate the brake pressing force f on thebasis of the pressing force signal, and calculate the desired value F*of the pressing force Fs of the disc brake 710 for the wheel inquestion, so that the braking forces for the front and rear wheels aresuitably distributed. Then, the control flow goes to step S603 toreceive the load signal from the load sensor 840, and calculate theactual pressing force Fs on the basis of the load signal. The controlflow then goes to step S604 in which a motor drive command signal forcontrolling the motor 772 is obtained on the basis of the calculatedactual pressing force Fs and the calculated desired value F*, and theobtained motor drive command signal is applied to the motor 772. As aresult, the motor 772 is controlled such that the pressing force Fs ofthe inner pad 720 b of the disc brake 710 for each wheel is equal to thedesired value F8. Thus, one cycle of execution of the brake controlroutine of FIG. 68 is terminated.

It will be understood from the above description of the twenty-fourthembodiment of this invention that the radial bearing 820 and the radialthrust bearing 822 which are adapted to receive at least the radial loadof the internally threaded member 802 are spaced apart from each otherin the axial direction of the internally threaded member 902, so as tominimize the inclination of the internally threaded member 802 withrespect to the body portion 761 of the caliper 736 and the motor housing780, even when the member 802 receives an offset load or unevenlydistributed load during activation of the disc brake 710. Thisarrangement is effective to minimize the local load concentration of theinternally threaded member 802 within the body portion 761 and motorhousing 780, which local load concentration would increase a resistanceto the rotary motion of the internally threaded member 802.

Further, the radial and radial thrust bearings 820, 822 are effective tominimize the inclination of the stator 784 with respect to the stator782 due to the offset load acting on the internally threaded member 802during activation of the disc brake 710, so that the oscillation of thestator 782 can be normally transmitted to the rotor 784, with a minimumamount of reduction of the drive torque of the motor 772 due to theabove-indicated inclination.

The present embodiment is further advantageous in that the thrust loadof the internally threaded member 802 is transmitted to the body portion761 through a reaction force transmitting portion in the form of theshoulder surface 826 and a reaction force receiving portion in the formof the shoulder surface 830. That is, the thrust force is nottransmitted from the internally threaded member 802 to the motor housing780.

The above arrangement makes it possible to improve the response of thedisc brake 710 to the operation of the ultrasonic motor 772, by simplyincreasing the rigidity of the body portion 761, without having toincrease the rigidity of the motor housing 780. Therefore, the motorhousing 780 may be made of a synthetic resin or may have a relativelysmall wall thickness, while permitting the disc brake 710 to have asufficiently high degree of operating response. In other words, it isnot necessary to increase the size and weight of the motor housing 780in order to improve the operating response of the disc brake 710.

The present embodiment is also advantageous in that the pad presserportion 736 a and the support portion 736 c are both integral parts ofthe caliper 736, resulting in a higher degree of rigidity of the caliper736 than where those portions 736 a, 736 c are separate members screwedto the caliper 736. In this respect, too, the operating response of thedisc brake 710 is improved.

The radial thrust bearing 822 used in the present embodiment is adaptedto receive both the radial load and the thrust load of the internallythreaded member 802. The use of this radial thrust bearing 822 makes itpossible to reduce the required number of the bearings, that is, to useonly two bearings, permitting the disc brake 710 to be manufacturedcompact at a reduced cost with reduced size and weight.

It will be understood that the shoulder surface 826 of the internallythreaded member 802 and the shoulder surface 830 of the support portion736 c cooperate with the radial bearing 820 and the radial thrustbearing 822 to constitute a rotary support mechanism for rotatablysupporting the internally threaded member 802. It will also beunderstood that the disc brake 710 has a first structure in which theinternally threaded member 802 is supported the support portion 736 cthrough the radial bearing 820 and the radial thrust bearing 822 whichare spaced apart from each other in the axial direction of theinternally threaded member 802. The disc brake 710 further has a secondstructure in which the stepped internally threaded member 802 issupported by the support portion 736 c through the radial thrust bearing822. It will further be understood that the elastic control mechanism740 constitutes the self-servo effect inhibiting means.

Referring next to FIGS. 69-73, there will be described twenty-fifththrough twenty-ninth embodiments of the present invention, which aresimilar in many aspects to the twenty-fourth embodiment but aredifferent from the twenty-fourth embodiment in the structure forsupporting the internally threaded member. The same reference signs asused in the twenty-fourth embodiment will be used in the embodiments ofFIGS. 69-73 to identify the commonly used elements, and the followingdescription refers to only the structures for supporting the internallythreaded member.

In the twenty-fifth embodiment of FIG. 69, the electrically operatedbraking system includes a motor-driven disc brake 860.

The disc brake 860 uses a thrust bearing 862 for receiving the thrustload of the internally threaded member 802, in place of the radialthrust bearing 822 used in the twenty-fourth embodiment. Further, thedisc brake 860 uses another thrust bearing 864 in addition to the radialbearing 820 used in the twenty-fourth embodiment. Like the radial thrustbearing 822, the thrust bearing 862 is disposed between the shouldersurfaces 826 and 830. On the other hand, the radial bearing 864 isdisposed between the rear portion 806 of the internally threaded member802 and the motor housing 780. In the present embodiment, the two radialbearings 820, 864 are provided on the opposite sides of the rotor 784.

In the present embodiment, too, the bearings 820, 862, 864 function tominimize the inclination of the internally threaded member 802 and therotor 784 during operation of the disc brake 860, and assure a highdegree of operating response of the disc brake 860 without increasingthe rigidity of the motor housing 780.

It will be understood from the above description of the twenty-fifthembodiment that the shoulder surface 826 of the internally threadedmember 802 and the shoulder surface 830 of the support portion 736 ccooperate with the radial bearings 820, 864 and the radial thrustbearing 862 to constitute a rotary support mechanism for rotatablysupporting the internally threaded member 802. It will also beunderstood that the disc brake 860 has a first structure in which theinternally threaded member 802 is supported the support portion 736 cthrough the radial bearings 820, 864 which are spaced apart from eachother in the axial direction of the internally threaded member 802. Thedisc brake 860 further has a second structure in which the steppedinternally threaded member 802 is supported by the support portion 736 cthrough the thrust bearing 862.

In the twenty-sixth embodiment of FIG. 70, the electrically operatedbraking system includes a motor-driven disc brake 870.

The disc brake 870 uses a radial bearing 872 in place of the radialbearing 820 used in the twenty-fourth embodiment. The radial bearing 872is disposed between the rear portion 806 of the internally threadedmember 802 and the motor housing 780.

In the present embodiment, the bearings 822, 872 function to minimizethe inclination of the internally threaded member 802 and the rotor 784during operation of the disc brake 870, and assure a high degree ofoperating response of the disc brake 870 without increasing the rigidityof the motor housing 780. Further, the required number of the bearingsis relatively small.

Further, the inclination of the internally threaded member 802 can bemore effectively prevented by the two bearings 822 and 872 which aredisposed near the opposite axial ends of the member 802, for receivingat least the radial load of the member 802.

It will be understood from the above description of the twenty-sixthembodiment that the shoulder surface 826 of the internally threadedmember 802 and the shoulder surface 830 of the support portion 736 ccooperate with the radial bearing 872 and the radial thrust bearing 822to constitute a rotary support mechanism for rotatably supporting theinternally threaded member 802. It will also be understood that the discbrake 860 has a first structure in which the internally threaded member802 is supported the support portion 736 c through the radial thrustbearing 822 and the radial bearing 872 which are spaced apart from eachother in the axial direction of the internally threaded member 802. Thedisc brake 860 further has a second structure in which the steppedinternally threaded member 802 is supported by the support portion 736 cthrough the radial thrust bearing 822.

In the twenty-seventh embodiment of FIG. 71, the electrically operatedbraking system includes a motor-driven disc brake 880.

Unlike the disc brake 710 of the twenty-fourth embodiment, the discbrake 880 is adapted such that the thrust load of the internallythreaded member 802 is transmitted through the motor housing 780 to thebody portion 761 of the caliper 736. The disc brake 880 uses a radialthrust bearing 882 in place of the radial thrust bearing 822 used in thetwenty-fourth embodiment. The radial thrust bearing 882 is disposedbetween the internally threaded member 802 and the motor housing 780.

The front portion 804 of the internally threaded member 802 has anintermediate part with a stop 884 formed on its outer circumferentialsurface. The stop 884 functions to limit the axial movement of theinternally threaded member 802.

In the disc brake 880, the bearings 820, 882 for receiving at least theradial load of the internally threaded member 802 are spaced apart fromeach other in the axial direction of the member 802, and thereforefunction to prevent the inclination of the member 802 and the rotor 784during operation of the disc brake.

Further, the radial thrust bearing 882 used in the present embodiment isadapted to receive both the radial load and the thrust load of theinternally threaded member 802. The use of this radial thrust bearing882 makes it possible to reduce the required number of the bearings,that is, to use only two bearings in the disc brake 880.

It will be understood that the radial bearing 820 and the radial thrustbearing 882 constitute a rotary support mechanism for rotatablysupporting the internally threaded member 802. It will also beunderstood that the disc brake 880 has a first structure in which theinternally threaded member 802 is supported the support portion 736 cthrough the radial bearing 820 and the radial thrust bearing 882 whichare spaced apart from each other in the axial direction of theinternally threaded member 802.

In the twenty-eighth embodiment of FIG. 72, the electrically operatedbraking system includes a motor-driven disc brake 890.

Unlike the disc brake 710 of the twenty-fourth embodiment, the discbrake 890 is adapted such that the thrust load of the internallythreaded member 802 is transmitted through the motor housing 780 to thebody portion 761 of the caliper 736. The disc brake 890 uses a radialbearing 892 and a radial thrust bearing 894 in place of the radialthrust bearing 822 used in the twenty-fourth embodiment. These bearings892, 894 are both disposed between the rear portion 806 of theinternally threaded member 802 and the motor housing 780. An annularspacer 896 is interposed between the bearings 892, 894.

In the disc brake 890, the bearings 820, 892 for receiving at least theradial load of the internally threaded member 802 are spaced apart fromeach other in the axial direction of the member 802, and thereforefunction to prevent the inclination of the member 802 and the rotor 784during operation of the disc brake.

It will be understood that the radial bearings 820, 892 and the radialthrust bearing 894 constitute a rotary support mechanism for rotatablysupporting the internally threaded member 802. It will also beunderstood that the disc brake 890 has a first structure in which theinternally threaded member 802 is supported the support portion 736 cthrough the radial bearings 820, 882 which are spaced apart from eachother in the axial direction of the internally threaded member 802.

In the twenty-ninth embodiment of FIG. 73, the electrically operatedbraking system includes a motor-driven disc brake 900.

Unlike the disc brake 710 of the twenty-fourth embodiment, the discbrake 900 is adapted such that the thrust load of the internallythreaded member 802 is transmitted through the motor housing 780 to thebody portion 761 of the caliper 736. The disc brake 900 uses a radialbearing 902 and a thrust bearing 904 in place of the radial thrustbearing 822 used in the twenty-fourth embodiment. The radial and thrustbearings 902, 904 are disposed at the between the front and rearportions 804, 806 of the internally threaded member 802, respectively.

In the disc brake 900, the bearings 820, 902 for receiving at least theradial load of the internally threaded member 802 are spaced apart fromeach other in the axial direction of the member 802, and thereforefunction to prevent the inclination of the member 802 and the rotor 784during operation of the disc brake.

It will be understood that the radial bearings 820, 902 and the thrustbearing 904 constitute a rotary support mechanism for rotatablysupporting the internally threaded member 802. It will also beunderstood that the disc brake 900 has a first structure in which theinternally threaded member 802 is supported the support portion 736 cthrough the radial bearings 820, 902 which are spaced apart from eachother in the axial direction of the internally threaded member 802.

In all of the twenty-fourth through twenty-ninth embodiments of theinvention, the two or more bearings for receiving at least the radialload of the internally threaded member 802 are spaced apart from eachother in the axial direction of the member 802, so as to prevent theinclination of the member 802 during operation of the disc brake.However, the same object may be achieved by using a single radialbearing wherein at least one of the outer and inner rings has aconsiderably large axial dimension.

While the present invention has been described in its presentlypreferred embodiments, for illustrative purpose only, it is to beunderstood that the present invention may be otherwise embodied.

In all of the illustrated embodiments, the disc brakes using theultrasonic motor or DC motor as the drive source are used as an ordinaryvehicle brake for braking the wheels during running of a vehicle, thedisc brakes may be used not only as the normal vehicle brakes but alsoas a parking brake for braking the vehicle to hold it stationary, or maybe used exclusively as the parking brake.

The techniques disclosed herein to overcome the drawbacks regarding theself-servo effect of the disc brake are applicable to both anelectrically or motor-driven operated disc brake and a mechanically orhydraulically operated disc brake.

It is to be understood that the present invention may be embodied withvarious other changes, modifications and improvements, which may occurto those skilled in the art, without departing from the scope of theinvention defined in the following claims.

What is claimed is:
 1. An electrically operated braking systemcomprising a motor-driven disc brake including an electric motor as adrive source for braking a wheel of an automotive vehicle, and a motorcontrol device for controlling said electric motor, said motor-drivendisc brake further including: a disc rotor having a friction surface androtating with said wheel; a friction pad movable for contact with saidfriction surface to restrict rotation of said disc rotor; a pad supportmechanism for supporting said friction pad such that said friction padis movable in a direction intersecting said friction surface; a padpressing mechanism comprising said electric motor and a pressing member,said electric motor producing a drive force for moving said pressingmember to force said friction pad against said friction surface of saiddisc rotor; a self-servo mechanism for providing a self-servo effect ofboosting a friction force generated between said friction surface andsaid friction pad, on the basis of the friction force; and a self-servoeffect inhibiting mechanism for inhibiting said self-servo mechanismfrom providing said self-servo effect while a braking force between saidwheel and a road surface is smaller than a predetermined first value,said self-servo effect inhibiting mechanism including means forinhibiting a movement of said friction pad due to said friction force.2. An electrically operated braking system according to claim 1, whereinsaid self-servo mechanism provides said self-servo effect by utilizingsaid movement of said friction pad with said disc rotor due to saidfriction force therebetween, such that said self-servo effect changeswith an amount of said movement of said friction pad, said self-servoeffect inhibiting mechanism including an elastic member whose elasticforce inhibits the movement of said friction pad with said disc rotor,said elastic force increasing non-linearly with an increase in an amountof elastic deformation of said elastic member.
 3. An electricallyoperated braking system according to claim 2, wherein a rate of increaseof said elastic force of said elastic member with said amount of elasticdeformation is higher when said amount of elas tic deformation isrelatively large than when said amount is relatively small.
 4. Anelectrically operated braking system comprising a motor-driven discbrake including an electric motor as a drive source for braking a wheelof an automotive vehicle, and a motor control device for controllingsaid electric motor, said motor-driven disc brake further including: adisc rotor having a friction surface and rotating with said wheel; afriction pad movable for contact with said friction surface to restrictrotation of said disc rotor; a pad support mechanism for supporting saidfriction pad such that said friction pad is movable in a directionintersecting said friction surface; a pad pressing mechanism comprisingsaid electric motor and a pressing member, said electric motor producinga drive force for moving said pressing member to force said friction padagainst said friction surface of said disc rotor; a self-servo mechanismfor providing a self-servo effect of boosting a friction force generatedbetween said friction surface and said friction pad, on the basis of thefriction force; and a mechanism including an elastic member formechanically controlling a rate of change of said self-servo effect ofsaid self-servo mechanism with a change in said drive force of saidelectric motor.
 5. An electrically operated braking system comprising amotor-driven disc brake including an electric motor as a drive sourcefor braking a wheel of an automotive vehicle, and a motor control devicefor controlling said electric motor, said motor-driven disc brakefurther including: a disc rotor having a friction surface and rotatingwith said wheel; a friction pad movable for contact with said frictionsurface to restrict rotation of said disc rotor; a pad support mechanismfor supporting said friction pad such that said friction pad is movablein a direction intersecting said friction surface; a pad pressingmechanism comprising said electric motor and a pressing member, saidelectric motor producing a drive force for moving said pressing memberto force said friction pad against said friction surface of said discrotor; a self-servo mechanism for providing a self-servo effect ofboosting a friction force generated between said friction surface andsaid friction pad, on the basis of the friction force; and a self-servoeffect inhibiting mechanism for inhibiting said self-servo mechanismfrom providing said self-servo effect while a braking force between saidwheel and a road surface is smaller than a predetermined first value,and wherein said self-servo-mechanism provides said self-servo effect byutilizing a movement of said friction pad with said disc rotor due tosaid friction force therebetween, such that said self-servo effectchanges with an amount of said movement of said friction pad, saidself-servo effect inhibiting mechanism including an elastic member whoseelastic force inhibits the movement of said friction pad with said discrotor, and wherein said pad support mechanism includes a stationarymember having a pair of portions for supporting said friction pad atopposite end portions thereof which are opposite to each other in arotating direction of said disc rotor, and said elastic member havingopposite end portions one of which is associated with one of saidopposite end portions Qf said friction pad toward which said frictionpad is moved with said disc rotor during forward running of saidautomotive vehicle, the other of said opposite end portions of saidelastic member being associated with one of said pair of portions ofsaid stationary member which is remote from said one end portion of saidfriction pad.
 6. An electrically operated braking system comprising amotor-driven disc brake including an electric motor as a drive sourcefor braking a wheel of an automotive vehicle, and a motor control devicefor controlling said electric motor, said motor-driven disc brakefurther including: a disc rotor having a friction surface and rotatingwith said wheel; a friction pad movable for contact with said frictionsurface to restrict rotation of said disc rotor; a pad support mechanismfor supporting said friction pad such that said friction pad is movablein a direction intersecting said friction surface; a pad pressingmechanism comprising said electric motor and a pressing member, saidelectric motor producing a drive force for moving said pressing memberto force said friction pad against said friction surface of said discrotor; a self-servo mechanism for providing a self-servo effect ofboosting a friction force generated between said friction surface andsaid friction pad, on the basis of the friction force; and an excessiveself-servo effect inhibiting mechanism for inhibiting said self-servoeffect inhibiting mechanism for inhibiting an increase of saidself-servo effect of said self-servo mechanism after a braking forcebetween said wheel and a road surface exceeds a predetermined secondvalue.
 7. An electrically operated braking system according to claim 1,wherein said motor-driven disc brake further includes temperature riserestricting means for restricting a rise of a temperature of saidelectric motor, said temperature rise restricting means comprising awaterjacket enclosing a housing of said electric motor and having afluid passage system extending therethrough, and further comprising apump operable to circulate a cooling fluid through said fluid passagesystem, for cooling said electric motor.
 8. An electrically operatedbraking system according to claim 1, wherein said self-servo mechanismincludes said friction pad which has a slant surface for contact withsaid pressing member, said slant surface having an inclination withrespect to said friction surface, an angle of said inclination of saidslant surface changing in a direction in which said friction pad ismoved with said disc rotor due to said friction force therebetween. 9.An electrically operated braking system according to claim 1, whereinsaid motor-driven disc brake includes a pair of friction pads disposedon opposite sides of said disc rotor, respectively, one of said frictionpads being movable with said disc rotor due to said friction forcetherebetween, while the other of said friction pads being immovable withsaid disc rotor due to said friction force, and wherein said padpressing mechanism includes a caliper extending over a periphery of saiddisc rotor and movable in said direction intersecting said frictionsurface, said caliper comprising a reaction portion engageable with saidone of said friction pads, and a presser portion for pressing said otherof said friction pads against said friction surface, said pad pressingmechanism further including a presser rod which is supported by saidpresser portion such that said presser rod is movable by said driveforce of said electric motor in said direction intersecting saidfriction surface, said caliper functioning as said pressing member forsaid one of said friction pads, while said presser rod functioning assaid pressing member for said other of said friction pads.
 10. Anelectrically operated braking system comprising a motor-driven discbrake including an electric motor as a drive source for braking a wheelof an automotive vehicle, and a motor control device for controllingsaid electric motor, said motor-driven disc brake further including: adisc rotor having a friction surface and rotating with said wheel afriction pad movable for contact with said friction surface to restrictrotation of said disc rotor; a pad support mechanism for supporting saidfriction pad such that said friction pad is movable in a directionintersecting said friction surface; a pad pressing mechanism comprisingsaid electric motor and a pressing member, said electric motor producinga drive force for moving said pressing member to force said friction padagainst said friction surface of said disc rotor; and a self-servomechanism for providing a self-servo effect of boosting a friction forcegenerated between said friction surface and said friction pad, on thebasis of the friction force, and wherein said electric motor has anon-energized off state, a first energized state for forward rotationthereof, and a second energized state for reverse rotation thereof, saidpressing member being moved to press said friction pad toward saidfriction surface of said disc rotor when said electric motor is placedin said first energized state, and wherein said motor control devicecontrols said electric motor such that an actual value of a pressingforce by which said friction pad is forced against said friction surfaceis equal to a desired value, said electrically operated braking systemfurther comprising: insufficient increase preventing means forpreventing a shortage of increase of said actual value of said pressingforce by locking said pressing member against a reaction forcetransferred from said friction pad to said pressing member, so as toprevent a shortage of increase of said actual value of said pressingforce, when said actual value is required to be increased duringoperation of said self-servo mechanism.
 11. An electrically operatedbraking system according to claim 10, wherein said electric motorconsists of an ultrasonic motor, and said motor control device comprisesde-energizing means for de-energizing said ultrasonic motor for therebyenabling said ultrasonic motor to produce a holding torque for lockingsaid pressing member, said insufficient increase preventing meanscomprising said de-energing means.
 12. An electrically operated brakingsystem according to claim 11, wherein said de-energizing means comprisesmeans for de-energizing said ultrasonic motor when an amount of increaseof said actual value of said pressing force is smaller than apredetermined first threshold while said ultrasonic motor is placed insaid first energized state.
 13. An electrically operated braking systemaccording to claim 11, wherein said de-energizing means comprises meansfor de-energizing said ultrasonic motor depending upon whether anoperation of said self-servo mechanism has been initiated.
 14. Anelectrically operated braking system according to claim 13, wherein saidmeans for de-energizing said ultrasonic motor depending upon anoperation of said self-servo mechanism has been initiated comprises asensor for detecting a value relating to said actual value of saidpressing force, and self-servo effect monitoring means for determining,on the basis of an output signal of said sensor, that the operation ofsaid self-servo mechanism has been initiated, if each of at least onepredetermined condition is satisfied, said at least one predeterminedcondition including a condition that an amount of increase of saidactual value of said pressing force exceeds a predetermined secondthreshold while said ultrasonic motor is placed in said first energizedstate.
 15. An electrically operated braking system comprising amotor-driven disc brake including an electric motor as a drive sourcefor braking a wheel of an automotive vehicle, and a motor control devicefor controlling said electric motor, said motor-driven disc brakefurther including: a disc rotor having a friction surface and rotatingwith said wheel; a friction pad movable for contact with said frictionsurface to restrict rotation of said disc rotor; a pad support mechanismfor supporting said friction pad such that said friction pad is movablein a direction intersecting said friction surface; a pad pressingmechanism comprising said electric motor and a pressing member, saidelectric motor producing a drive force for moving said pressing memberto force said friction pad against said friction surface of said discrotor; and a self-servo mechanism for providing a self-servo effect ofboosting a friction force generated between said friction surface andsaid friction pad, on the basis of the friction force, and wherein saidelectric motor includes a stator, a rotor and a motor housing in whichsaid stator and said rotor are accommodated, and said pad pressingmechanism includes: a rotatable member rotatable about an axis thereofby said electric motor; a linearly movable member disposed rearwardly ofsaid pressing member such that said movable member is linearly movablein said direction intersecting said friction surface of said disc rotor;a motion converting mechanism for converting a rotary motion of saidrotatable member into a linear motion of said linearly movable member,to move said pressing member for forcing said friction pad against saidfriction surface; a caliper including a portion functioning as saidmotor housing, and supporting said linearly movable member such thatsaid linearly movable member is linearly movable; and a rotary supportmechanism for supporting said rotatable member rotatably relative tosaid caliper, said rotary support mechanism enabling said caliper toreceive as a thrust load a reaction force from said rotatable memberwhile said friction pad is forced against said friction surface, andwherein said rotary support mechanism includes a support structure forreducing an influence of at least one of a first reaction force and asecond reaction force upon said electric motor, said first reactionforce being received as an offset load by said rotatable member fromsaid linearly movable member during an operation of said motor-drivendisc brake, and said second reaction force being received by saidcaliper from said rotatable member during the operation of saidmotor-driven disc brake, and further wherein said support structureincludes a first structure for restricting an inclination of said axisof said rotatable member by said first reaction force during theoperation of said motor-driven disc brake.
 16. An electrically operatedbraking system according to claim 15, wherein said rotatable member iscoaxially fixed to said rotor for rotation therewith, and said firststructure includes a structure for restricting the inclination of theaxis of said rotatable member to thereby restrict an inclination of anaxis of said rotor with respect to an axis of said stator.
 17. Anelectrically operated braking system according to claim 15, wherein saidfirst structure includes a plurality of radial bearings for rotatablysupporting said rotatable member, said radial bearings being spacedapart from each other in an axial direction of said rotatable member andreceiving a radial load from said rotatable member.
 18. An electricallyoperated braking system according to claim 15, wherein said supportstructure includes a second structure inhibiting said second reactionforce from being transmitted to said electric motor.
 19. An electricallyoperated braking system according to claim 18, wherein said rotatablemember has a first surface which faces in an axial direction of saidrotatable member from said friction pad toward said rotatable member andwhich transmits said second reaction force to said caliper, and saidcaliper has a second surface formed at a portion thereof between saidportion thereof functioning as said motor housing and a portion thereofcorresponding to said first surface, said second surface being opposedto said first surface in the axial direction of said rotatable memberand receiving said second reaction force from said first surface, saidsecond structure including said first and second surfaces and a bearingwhich is interposed between said first and second surfaces and betweensaid rotatable member and said caliper such that said bearing rotatablysupports said rotatable member so as to receive at least a thrust loadfrom said rotatable member.
 20. An electrically operated braking systemaccording to claim 1, wherein said self-servo effect inhibitingmechanism inhibits an operation of said self-servo mechanism while afriction force between said friction pad and said friction surface ofsaid disc rotor is smaller than a predetermined threshold.
 21. Anelectrically operated braking system according to claim 1, wherein saidself-servo effect inhibiting mechanism inhibits an operation of saidself-servo mechanism while a deceleration value of said automotivevehicle braking braked by said motor-driven disc brake is smaller than apredetermined threshold selected within a range of 0.5-0.6 G.
 22. Anelectrically operated braking system according to claim 1, wherein saidself-servo effect inhibiting mechanism inhibits an operation of saidself-servo mechanism while an amount of operation of a brake operatingmember manually operable to operate said motor-driven disc brake issmaller than a predetermined threshold.
 23. An electrically operatedbraking system according to claim 1, wherein said motor-driven discbrake further includes temperature rise restricting means forrestricting a rise of a temperature of said electric motor, saidtemperature rise restricting means comprises a cooling fan for blowingair toward said electric motor.